Scottish home cooking has always been about making a great deal from a short list of good things. Oats in the kist, a piece of cheese, a few leeks pulled from the winter rig — that trio has fed families here for centuries, and it still makes sense. This tart folds all three into one dish: sturdy enough to carry in a lunch box, handsome enough for a Sunday table, and ready in well under an hour once the pastry has had its rest.

The crust is the part that raises eyebrows. It is not a delicate, flaky shortcrust but something nearer a savoury oatcake pressed into the tin — coarse, nutty and faintly crumbly, with just enough butter to hold it together. The filling is soft, milky and mild, sharpened with mustard and nutmeg. Served warm with a sharply dressed salad, it is one of the most useful things you can bake on a grey afternoon.

Why oatmeal belongs in the pastry

Medium oatmeal gives the crust a texture that plain flour alone cannot manage. It is slightly gritty in the best way, and toasts to a warm, biscuity flavour in the oven. Use too much and the base turns sandy, so the trick is a roughly equal split with plain flour — around 150g of oatmeal to 100g of flour for a 23cm loose-bottomed tin.

Two practical points matter more than the recipe itself. First, press the mixture into the tin rather than rolling it; oatmeal pastry tears easily and has no gluten structure to forgive rough handling. Second, chill the lined tin for at least fifteen minutes before baking. A cold crust holds its shape and shrinks far less. A tablespoon of pinhead oats stirred through gives extra crunch, and a little cold butter rubbed in with your fingertips, or pulsed briefly in a processor, is all the work required.

Choosing the cheese: crowdie, cheddar, or both

Crowdie is a fresh, soft Scottish cheese, lightly soured and milky, somewhere between cottage cheese and cream cheese in character. It makes a beautifully light filling, but it will not set firmly on its own, so it needs eggs and either a little cheddar or a spoonful of cream to give it body.

• All crowdie: drain it well in a sieve for half an hour first, then use 300g with three eggs. The set will be soft and delicate.
• A half-and-half mix: 150g crowdie and 100g grated mature cheddar. This is the version I make most often — tangy, well seasoned and reliably firm.
• All cheddar: 200g of a mature Scottish cheddar, grated. Stronger, saltier and excellent with a spoonful of wholegrain mustard.

If you are cooking for vegetarians, check the label for vegetarian rennet. Most supermarkets now stock suitable crowdie and cheddar, but small farmhouse cheeses often use animal rennet, and it is worth a moment's glance.

Getting the leeks right

Leeks hold grit deep between their layers, so slice them lengthways, fan out the leaves under cold running water, then slice thinly across. Two good-sized leeks, about 300g trimmed, will fill the tart properly.

Sweat them gently in 40g of butter for ten to twelve minutes with a pinch of salt. You want them soft, sweet and glossy, not coloured — browning brings a bitterness that fights the cheese. Let them cool completely before they meet the eggs, or they will start to cook the custard in the bowl. A grating of nutmeg, a pinch of mustard powder, plenty of black pepper and a few thyme leaves are the only other flavourings needed.

The tart, step by step

• Pastry: 150g medium oatmeal, 100g plain flour, 125g cold butter, a good pinch of salt, one egg yolk and a splash of iced water.
• Filling: 300g leeks, 40g butter, 150g drained crowdie, 100g grated mature cheddar, 3 eggs, 150ml double cream, mustard powder, nutmeg, pepper.

• Rub the butter into the oatmeal, flour and salt until it resembles coarse breadcrumbs, then bind with the yolk and enough water to bring it together. Add the water a teaspoon at a time.
• Rest the dough in the fridge for 30 minutes, then press it evenly across the base and up the sides of a 23cm loose-bottomed tart tin. Chill again for 15 minutes.
• Heat the oven to 190°C fan, 200°C conventional, gas mark 6. Line the pastry with baking paper and baking beans and bake for 15 minutes. Remove the paper and bake for a further 5 minutes, until the base looks dry.
• Meanwhile, sweat the leeks in the butter until soft, then cool. Whisk the eggs with the cream, stir in both cheeses and the seasonings, then fold through the leeks.
• Pour into the case, smooth the top and bake for 30 to 35 minutes, until set with the faintest wobble in the centre.
• Cool in the tin for 15 minutes before releasing the ring. Serve warm or at room temperature.

Serving, keeping and ringing the changes

A sharply dressed salad of bitter leaves, a spoonful of pickled beetroot and a few oatcakes on the side make a complete light lunch. The tart also travels well, which is why it turns up in so many picnic baskets: wrap it cold, slice it thick and it will hold together beautifully.

It keeps for three days covered in the fridge and reheats gently in a low oven, though I think it is best at room temperature. Individual slices freeze well; wrap them separately and thaw overnight. For variations, try a handful of chopped kale wilted with the leeks, a scattering of toasted walnuts, or a smoked cheese in place of the cheddar — though that last one will need a lighter hand with the salt. Whatever you do, do not rush the cooling. A tart cut hot slumps on the plate, and this one deserves better.