A proper cottage pie is one of the great pillars of British home cooking — deeply savoury, generous with the mash, and baked until the edges catch and crisp. Take the mince out of the equation, though, and you need something with enough body to carry all that flavour. Green lentils do exactly that. They hold their shape through a long, slow simmer, they drink up stock and root vegetables, and they give the filling the same dense, spoonable texture we expect from the traditional version.

This one is built around what turns up in the veg box between October and March: carrots, parsnips, a couple of celery sticks and a good onion, under a thick blanket of buttery mash. It feeds six comfortably, costs very little, and — this matters on a wet Wednesday — reheats beautifully the next day.

Why Lentils Beat Meat-Free Mince Here

Shop-bought meat-free mince is fine in a ragu, but in a pie it can turn spongy and release water into the sauce. Dried green or brown lentils are far better behaved. Buy the small khaki ones sold simply as "green lentils" in most supermarkets; red split lentils are wrong for this job, as they collapse into purée and are better suited to soups and dal.

You don't need to soak them. Rinse them well and pick over for the occasional small stone, then simmer gently. Puy lentils are excellent — a little firmer and earthier — but pricier. Either works, and leftovers are superb.

One warning worth repeating: traditional Worcestershire sauce contains anchovies. If you're cooking for vegetarians, use a vegetarian Worcestershire-style sauce, or a splash of soy sauce instead.

The Root Vegetables, and How to Cut Them

Carrots and parsnips are the backbone of this pie, and parsnips are at their sweetest after the first frosts. Cut both into a 1–1.5cm dice rather than coins. Round slices stay firm and slightly waxy in the sauce, while dice soften into the gravy and thicken it as they go.

A few other roots work well in place of either, depending on what you have:

• Swede — a third of a medium one, diced small, adds a lovely peppery note.
• Celeriac — the top half of one, diced, gives a gentle celery flavour that suits lentils.
• Jerusalem artichokes — a handful, diced, for a nutty, slightly smoky depth.

Keep a bag of frozen peas in the freezer and stir a couple of handfuls in at the very end, once the filling is off the heat.

Building the Filling

This is the part that decides whether your pie is a triumph or a puddle. Take your time with the base.

• 3 tbsp rapeseed or olive oil
• 1 large onion, finely chopped
• 2 celery sticks, finely chopped
• 2 carrots and 2 parsnips, cut into 1–1.5cm dice
• 3 cloves garlic, sliced
• 1 tbsp tomato purée
• 1 tsp dried thyme, or 4 sprigs of fresh
• 200g dried green lentils, rinsed
• 1.2 litres hot vegetable stock
• 1 tbsp vegetarian Worcestershire-style sauce
• Salt and plenty of black pepper

Soften the onion and celery in the oil over a low heat for 10 minutes without colouring them. Add the carrot and parsnip and cook for another 5 minutes, then stir in the garlic, tomato purée and thyme and let them catch for a minute. Tip in the lentils, pour over the hot stock and add the sauce. Bring to a steady simmer, then cook uncovered for 30–35 minutes, stirring now and then, until the lentils are tender and the liquid has reduced to a thick gravy that holds its shape on a spoon.

If it looks loose, simmer a few minutes longer; if it looks dry, add a splash more stock. Season aggressively — lentils need more salt than you think — then leave it to cool slightly while you deal with the potatoes.

Mashed Potato: The Topping That Makes It

Use 1.2kg of floury potatoes — Maris Piper, King Edward or Kerr's Pink. Peel and cut them into even chunks, cover with cold salted water and boil for 18–20 minutes until a knife slides in with no resistance. Drain, then leave them in the colander for two or three minutes to steam dry. This one step is the difference between fluffy mash and wallpaper paste.

Mash or rice them, then beat in 60g butter and about 100ml of warm milk, a teaspoon of mustard and a grating of nutmeg. Don't use a food processor; it will turn the starch gluey. Season, taste, and season again.

Assembling, Baking and Getting the Crust Right

Heat the oven to 200°C (180°C fan, Gas Mark 6). Tip the filling into a 2–2.5 litre ovenproof dish. Spoon the mash over in rough blobs, spread it gently with a fork — don't press it flat — and fork the surface into ridges so the peaks catch. Dot with a little more butter and bake for 25–30 minutes until golden and bubbling at the edges. If it needs more colour, three or four minutes under a hot grill will do it, but stay by the oven.

Let it rest for 10 minutes before serving; the filling is molten straight from the oven. Buttered greens or braised red cabbage are the classic partners.

Seasonal Tweaks and Making It Ahead

In spring, add frozen broad beans with the peas and a little lemon zest at the end. In autumn, swap one parsnip for diced squash. In deepest winter, a spoonful of redcurrant jelly or a splash of dark ale in the stock gives the filling a wonderful richness.

The pie freezes well. Assemble it in a freezer-proof dish, cool completely, cover and freeze for up to three months. Bake from frozen at 170°C fan, covered with foil, for 50 minutes, then uncovered for 25 minutes more. Reheat leftovers in the oven rather than the microwave — you'll lose the crust otherwise, and the crust is half the point.