Carla Tomasi is one of the few people I call on the phone. She is also the person who, more than anyone, has taught me about cooking vegetables. There is a very good chance that she will have her hands in the soil of her marvellous vegetable garden when I call; her phone flips to voicemail, then she calls back. Today, it was the soil in the pots lining her covered veranda as the rain pummelled her thirsty garden near Ostia, about 30km outside Rome. The plants got some welcome rain, and I got a welcome recipe for her mushroom and potato bake, which I wrote on a Post-It note and stuck to my desk about a month ago; the fluorescent yellow answer just waiting for the perennial question: “What shall we have for dinner?”

Some recipes require precision, while others require generously defined principles and good advice – and this is always the case with Carla’s vegetable dishes, and in particular her potato and mushroom bake.

The mushrooms are the heart of the matter, so they must be well seasoned and full of flavour. The variety of potato matters here: they shouldn’t be too floury or waxy. Desiree are ideal, Carla notes during our phone call, the heavy rain punctuating our conversation like a crossed line. It is important to slice the potatoes thinly; I do this on the slice side of a box grater, three mouth-like slats being the nearest thing I have, and indeed want, to a mandoline. Unless you are careful, box-grater slats rarely produce whole slices but rather half and three-quarter moons, which are fine.

This is good eaten hot, even better just warm. At room temperature, it becomes more compact, with the cheese acting as a flavoursome glue, making it easier to slice. Serve with a green salad, red-wine vinaigrette and some braised spring vegetables (peas, broad beans, and courgettes perhaps). This is the kind of cooking I like: everyday ingredients put together with style. It’s cooking that doesn’t require too much precision, but benefits from care – and some good advice from a friend.

Carla’s potato and mushroom bake

After step two, you may well be tempted to pile the mushrooms – something the Italians refer to as funghi trifolati – on to a piece of toast or stir through pasta – both of which are excellent.

Prep and soak 30 min

Cook 1 hr 10 min

Serves 4 as a main, 6 as a starter

A few dried porcini, soaked for 30 minutes in 100ml hot water.

Butter

500g mushrooms

3 tbsp olive oil

1 garlic clove, minced

1 tbsp chopped parsley

1kg potatoes – all rounders, not too floury or waxy

60g parmesan, grated

Soak the porcini in 100ml hot water for half an hour. Butter the bottom and sides of a shallow 25cm cake tin. Put a circle of baking parchment in the bottom and butter that, too. Heat the oven to 180C (160C fan)/350F/gas 4

Slice the mushrooms. In a frying pan, warm the olive oil, a walnut of butter, and the garlic, then add the mushrooms and a pinch of salt. As they cook and exude juice, raise the heat a bit so that by the time the mushrooms are soft and collapsed, there is just a little liquid remaining. Drain and chop the porcini and add to the pan, along with the parsley.

Peel and slice the potatoes thinly – I do this on the slicing side of a box grater. Put in a bowl, season with salt and two tablespoons of olive oil, then toss.

Arrange half the potato in the bottom of the tin as neatly or hastily as you like. Cover with the mushrooms and almost all of the grated parmesan. Then cover with the rest of the potatoes, press down, and dot with butter. Cut a round of baking parchment and press on top to make a cover.

Bake at the bottom of the oven for 10 minutes, move to the middle of the oven for 30 minutes, then turn the oven up to 200C (180C fan)/390F/gas 6 for the last 10 minutes of cooking, to get a golden bottom.

Remove from the oven, set aside for 10 minutes, then run a palette knife around the edge to loosen the bake. Now invert it on to a plate and pull away the parchment.

The bottom – now the top – should be golden and crusty. If it isn’t, sprinkle with a little more grated parmesan and slide under the grill for a few minutes so it bubbles and browns.