Brown the turkey: Heat the olive oil in a large frying pan over a medium-high heat. Add the turkey mince, breaking it up with a wooden spoon. Cook for 5-7 minutes until it is golden brown all over and no pink remains. The sizzling sound will quieten as the moisture evaporates, and you’ll see the meat turn a rich, speckled brown. Transfer the browned turkey to your slow cooker.

Sweat the vegetables: In the same pan, add the chopped onion, carrots, and celery. Cook over a medium heat for 5-6 minutes, stirring occasionally. The onions will turn translucent and release a sweet, fragrant smell. Add the minced garlic and tomato purée, then stir for another minute until the garlic is aromatic and the purée darkens slightly.

Make the gravy base: Sprinkle the flour over the softened vegetables and stir continuously for 1-2 minutes. This cooks out the raw flour taste and creates a paste that will thicken our gravy. The mixture will look clumpy and dry at first, but that’s exactly what we want.

Deglaze and combine: Slowly pour in the warm stock while stirring constantly. Add the Worcestershire sauce and dried thyme. Bring the mixture to a gentle simmer; you’ll see it thicken to a glossy, rich gravy that coats the back of a spoon. Pour this entire mixture into the slow cooker with the turkey. Stir well to combine.

Slow cook the filling: Cover the slow cooker with the lid and cook on low for 6-8 hours, or on high for 3-4 hours. The filling should be bubbling gently around the edges and smell deeply savoury. About 30 minutes before the end of cooking, stir in the frozen peas. They will warm through and retain their bright green colour and slight sweetness.

Prepare the mashed potato topping: While the filling cooks, place the quartered potatoes in a large saucepan and cover with cold salted water. Bring to a boil, then reduce the heat and simmer for 15-20 minutes. You’ll know they’re done when a knife slides through the centre of a potato piece with no resistance. Drain them thoroughly in a colander.

Mash and season: Return the potatoes to the warm pan (or a warm bowl) to steam off any excess moisture for a minute. Add the butter and begin mashing. Slowly pour in the warm milk, mashing until you achieve a smooth, creamy consistency. Season with salt and pepper. If using, stir in the grated cheddar cheese until it melts into the mash, giving it a lovely, golden finish.

Assemble and finish: Once the filling is cooked, spoon the mashed potato over the top. Use a fork to create a swirl pattern on the surface; those peaks will turn beautifully crisp under the grill. If you have an air fryer or oven-safe slow cooker insert, you can place the whole thing under a hot grill for 5-7 minutes until the potato is golden and slightly crunchy on the edges. Otherwise, serve it straight from the pot — the flavour is just as good.