Put the phrases “exclusive members club” and “clean eating” together and you can all but guarantee an unusual evening. So it proves at Searcys – an upmarket drinking and dining club in the heart of the City of London. But if you are imagining a dated, oak-panelled establishment that does a good line in salads, think again.

Searcys is something else entirely. Occupying the top two floors of London’s Gherkin skyscraper, it is slick, suave and sophisticated, offering its well-heeled members bar and restaurant facilities at altitude. The club opens its doors to the public only for special occasions, including sporadic pop-ups and collaborations.

The latest of these, and the reason I’m here, is a “Saints and Sinners” menu devised to test even the most ardent new-year dieters. It’s the unusual brainchild of two rather different cooks: the club’s classically-trained executive chef, Barry Tonks, whose spell as head chef at McClements in Twickenham saw it awarded a Michelin star, and Alice Liveing, better known as Clean Eating Alice - a health food guru who shot to prominence with an Instagram page that now has a loyal following of half a million.

The pop-up sees recipes from Liveing’s latest book, Eat Well Every Day, given a contemporary twist by Tonks and dished up as part of a split menu that includes her array of lighter, healthier ‘Saint’ dishes opposite Tonks’ sumptuous but sinfully calorific concoctions.

I am, I must admit, no expert on the clean eating trend, generally preferring a KitKat to kale and steak to a spiraliser. It is with some suspicion, then, that I turn up at Searcys on a Friday night imagining a potentially majestic dining experience devastated by a deluge of radish and lettuce and dished up with an avocado smoothie and a colonic irrigation voucher.

Chef Barry Tonks and Clean Eating Alice (Alice Liveing) make an unusual pairing

My misconceptions are quickly cast aside, however, as the lift rockets us up to the 39th floor, where we emerge onto the flight deck. Or, as Searcys call it, the restaurant. The décor is sleek and shiny, with sparkling black-tiled floors and steel furniture giving a futuristic, metallic vibe that fits the style of the building and creates the impression of being on a spacecraft hovering above the city. Look up and you see the sides of the so-called Erotic Gherkin sloping towards the centre to form its famously phallic shape.

Frills are kept to a minimum to avoid distracting from the floor-to ceiling windows that provide the restaurant’s main draw in the form of stunning, sweeping vistas of the city skyline, from the Shard to the south down to Tower Bridge looking like a child’s toy dumped across the river winding way below. It is not as deep in the clouds as restaurants in the Shard or the Heron Tower but nevertheless is up there – quite literally – with the best dinner views in London.

When it comes to eating, you could be forgiven for diving straight in to the Sinner menu – a tempting choice when it includes foie gras, braised Ox cheek, butter-roasted halibut and tart tatin. But that, I am to discover, would be a mistake, for the Saint side also has much to offer.

Diners are free to mix and match from the two menus so my dining partner and I opt for a selection of both Saint and Sinner dishes. To start, a plate-sized slice of aubergine arrives smeared with half-melted feta and drizzled with pomegranate seeds - perfectly pleasant but not the sort of dish that leaves you wanting more. It would work as a light lunch in an Islington organic café but disappoints as a standalone dinner course.

The lobster ravioli, from the Sinner menu, is a different matter. The parcel of silky pasta encases a good quantity of succulent white lobster meat and is smothered, deliciously, in a yellow, flavour-packed lobster and Armagnac sauce that tastes like all your favourite seafood blended into one.

The exclusive members club only opens its doors to the public on special occasions

Between courses, the club’s suave French general manager, Karim LeCloarec, flits between tables offering expert advice on wines for each course and information on where they come from – understanding origins, so I’m told, being a key part of the clean eating concept. Even the booze has been given a healthy twist. We start with a glass of unusually cloudy prosecco that is “biodynamic”, so no sulphites have been added and pesticides on the vines are kept to a minimum. It’s crisp, lightly sparkling and deliciously smooth and after a couple of glasses I am becoming more convinced by this clean eating lark.

“It’s actually the sulphites in the wine that give you a hangover – not the wine itself”, Karim confides, in what strikes me as a potentially life-changing revelation. I’m about to offer to test his theory with another glass or two but before I know it he’s recommending a French chenin blanc and telling me about this year’s wine trends in the Loire Valley. Searcys does have a dedicated sommelier, but with Karim around, it is not entirely clear why.

The mains take things up several notches. The sumptuous parmesan and truffle risotto is creamy and tangy and everything a good risotto should be. Portion sizes are just right and the mass of cheesy, gooey goodness contains a decent amount of earthy Perigord truffle shavings.

It is eclipsed, though, by the expertly-braised venison chunks drizzled with a delicate, burgundy-coloured jus. Tender, succulent and packed with flavour, it is a delight - and an apparently ‘saintly’ one at that. The meat is accompanied by some beautifully sticky, herby, miso-encrusted parsnips and a smattering of pomegranate seeds. A side of rich, creamy dauphinoise and a green salad top it off nicely.

The tart tatin comes with its crust covered in crispy, crunchy caramel and topped with apples that melt in the mouth

There is barely room for dessert but, in the interests of journalistic rigour, we selflessly plough on. It’s a good move. A slab of tart tatin comes with its crust covered in crispy, crunchy caramel and topped with apples that melt in the mouth. With it, unusually, is served not a dessert wine but a pear cider. I am initially sceptical, fleetingly questioning the culinary value of a glass of Strongbow with my dessert, but this is something else entirely. The mouth-wateringly tangy cider, like none I’ve ever tasted, is made up of 20 different types of pear and perfectly offsets the cloying sweetness of the caramel. It’s an unusual choice but a genius one, and the combination works a treat.

My companion opts for the fruity French toast –classed, slightly surprisingly, as a Saint dish. Topped with fresh berries and a side of frozen yoghurt, it’s a light, tasty affair – the tartness of the blackberries complementing the slight sweetness of the egg-friend bread. Again, though, it’s the Sinner dish that comes up trumps, and I’m happily picking away at the tart tatin as Karim arrives to impart another lesson in global wine trends.

In truth, the slightly forced “Saint and Sinner” concept distracts from what, whichever side of the menu you order from, turns out to be a very good meal in an even better setting - even if some of Liveing’s lighter dishes are eclipsed by the devilish but delicious depth of Tonk’s own creations. While the evening skyline is impressive, a lunchtime visit is a tad cheaper and would offer more extensive views across the capital. Still, as the lift plummets us back down to earth in a bloated and slightly wine-fuelled haze, I can’t help thinking that clean eating can taste really rather good – especially, I’ll admit, when it’s offset by generous lashings of lobster, truffle and apple tart.

Food: ****

Ambience: ***

Service: *****