In the US, nothing says “grandiose occasion” like immersing food in hot fat. Their state fairs – think enormous village fetes – regularly set arteries racing with the likes of deep-fried butter, deep-fried bubblegum and lashings of neon-pink Kool Aid (also deep-fried). Patriotically bathing a whole Thanksgiving turkey in oil is so popular that antsy fire departments take to the airwaves to warn people that, while the upside is a nice crispy mouthful, the potential downside is a family gathering that looks like Backdraft. With the exception of the battered antics of the odd Scottish chippy, the Brits don’t really do that sort of thing.

Fry Hard, a pop-up restaurant coming to London’s Shoreditch’s (where else?) plans to change this. They will commemorate Easter on “Good Fry Day” by offering a whole deep-fried lunch. Could roast lamb and all the trimmings really be improved by being battered and immersed in fat? I joined them in their test kitchen to find out.

“Let’s try deep-frying gravy,” suggests Jim Thomlinson, a chef whose previous gigs include working at Michelin-starred restaurant The Glasshouse. “You’ll be able to pop it so that all the juice comes out inside your mouth,” explains business partner Emma Thomas – AKA Miss Cakehead, a “freelance creative director” who makes a living from organising headline-gathering culinary stunts. She pulls out a block of jellified stock and hands it to Thomlinson for dunking in batter. As it hits the oil, there’s a hiss like a cat seeing its own reflection and a forboding “Ah” from Thomlinson. Given that the oil is now turning so brown you could patent its shade as post-Marbs TOWIE star, this hasn’t been a success.

Believe it or not, there are some things whose flavour isn’t improved by being dunked in boiling oil. Photograph: Graeme Robertson

“I guarantee that the roast will work, though,” says Thomlinson, deftly battering belly lamb, yorkshire puddings, roast potatoes, root veg and greens. And he should know – he’s spent the last month assembling a 100-strong menu of deep-fried items including fried Battenberg, gherkins and Turkish delight, and testing classics such as Mars bars, Snickers and Creme Eggs. Which, I suppose, is a case of batter the devil you know.

The potatoes are lowered in carefully, the Yorkshire puddings bob up and down, rims breaking the surface like little pastry lifesavers. The small-ish pieces of lamb belly are busily transmogrifying into golden-brown fists of meat. But none of the ingredients have anything on the haricots verts. “These are my absolute favourites,” grins Thomlinson as we watch beans bubble up into glistening, coral-like tendrils.

Just 10 minutes later, it’s plated up and – lest the litres of fat didn’t tip you off – one glance tells you that it’s unlikely to have the British Nutrition Foundation singing the theme tune to Happy Days. Partly because the ingredients have swollen into greasy batons and balls so sturdy that they could form the components of a slippery lawn game. But mainly because of the colour. If keeping your heart happy involves picking enough different coloured ingredients that you “eat the rainbow”, I’m in trouble here. The thing is a visual symphony of browns. It is browned off. It is Mrs Brown’s boys on a brownfield site singing Brown Eyed Girl.

But boy does it taste good. “Mmm, that’s ridiculously crunchy,” exclaims Thomlinson as we tuck into a portion of potatoes that are all light, brittle crust and soft, floury centres. The parsnips are hyper-sweet tempura and the pink, pert hunks of lamb belly are moist beyond belief, albeit much in need of the mint sauce – hardly surprising given that this already greasy meat now features so much extra oil that an emir could base an economy on it. The only real misfire is the Yorkshire puddings: the thirsty little sods have slurped up half the contents of the pan so that biting into them is like chowing down on a sponge that’s fallen into a grease trap.

Pretty soon, though, we’re not feeling so chipper. “I need a nap,” groans Thomas. I’m feeling like I should check myself into a cardiology ward. Even Thomlinson, who’s bearing up surprisingly well, is mumbling: “Someone will be not very well if we serve them this.” A quick calculation later and the mistake is realised. Without batter, this was a meal for one; battered and fried, it’s a heavy portion that would fill four. We should have figured that out earlier, really. After all, this dish was no small fry.

Four things that don’t deep fry

Believe it or not, there are some things whose flavour isn’t improved by being dunked in boiling oil.

Deep-fried Toblerone ... “inedible”. Photograph: Graeme Robertson

Toblerone

This mountain range of artery-hardening goop should work in theory. “But it doesn’t,” says Thomlinson. The chocolate is so thin that it just melts into the deep fat fryer until all that’s left is this weird shell. It’s inedible.”

Cornetto

Fancy a deep-fried ice-cream? “You can’t use a Cornetto,” says Thomlinson, looking sadly at a hole-pocked batter carapace that weeps vanilla tears. “It’s soft serve ice-cream – designed to be eaten straight out of the freezer. You need your ice-cream to be really hard for it to not melt.”

“Totally minging” ... deep-fried lettuce. Photograph: Graeme Robertson

Lettuce

The perfect snack. If your ideal foodstuff smells like a McChicken sandwich that’s been left in a bin and tastes like vegetable oil glugged straight from the bottle. “The oil just oozes into your mouth,” says Thomas. “It seeps into all the layers of the lettuce.” “Minging,” adds Thomlinson, lest there be doubt. “Totally minging.”

Nutella

Oozy chocolate is one thing. But surrounding it with a coating whose texture is similar to suet? “It doesn’t work,” offers Thomlinson. “Because Nutella is basically a liquid, it penetrates the batter. There’s just no crunch.”

Fry Hard’s Sunday roast cost £20 and have to be ordered in advance via Facebook, @LDNmessChef or @Miss_Cakehead.