Preheat the oven to 200°C/180°C fan. You will need an ovenproof dish approx. 20x30cm.

If you are making the sweet potato mash from scratch, peel and dice the sweet potatoes and boil in a saucepan of salted water for 10-15 minutes, or until completely tender. Drain and mash, adding a splash of olive oil, some salt and pepper. If you're using shop-bought mash, warm gently in the microwave before using.

Warm the olive oil in a large large frying pan set over a medium heat. Add the diced onion, carrot and chopped garlic and cook for 5-10 minutes, until softened but not browned. Add the ground cumin, coriander, tomato puree, a big pinch of salt and some black pepper, and cook for 2-3 minutes, until fragrant.

Next, add the cooked green lentils and their water, and the Puy lentils. Fill the empty lentil can with water and add this too (crumble in a veggie stock cube if you have one to hand). Bring to the boil, then reduce the heat and simmer for 10 minutes, until reduced by a third.

Once the sauce has reduced stir in the baby spinach and taste to check the seasoning. Pour the lentil mixture into the baking dish and spoon the sweet potato mash over the top, using the back of a spoon to make swirls in the potato, this will create crispy edges. Scatter over the grated cheddar (if using).

Cook for 30 minutes, until golden and bubbling. Leave to stand for 10 minutes before serving (if you can wait!).