Heat your butter and oil in a casserole pan, and then add the onions, carrot, and celery. Sauté for 6-7 minutes until the vegetables turn fragrant and begin to soften.
1 tbsp olive oil, 1 tbsp unsalted butter, 1 large white onion, 1 medium carrot, 1 celery stick
Meanwhile, make the mushroom stock by pouring about 500ml boiling water over the stock cube and dried porcini mushrooms in a jug, and stir. Leave to steep until you need the stock - remove the porcini mushrooms and finely dice them before adding to the pan at the same time as adding the mushrooms.
20 g dried porcini mushrooms, 1 vegetable stock cube
Add the garlic, mushrooms, and seasonings. Cook for another 5-8 minutes and then add the flour and stir it into the vegetables, cooking off for a minute or two.
2 cloves of garlic, 250 g chestnut mushrooms, 1 tsp thyme or 2 sprigs fresh thyme, 2 tbsp plain flour, Salt and pepper to taste
Add the puy lentils.
250 g cooked puy lentils
Pour in the mushroom stock and stir through the cream cheese and the marmite, leaving to simmer on low heat while you prep the vegetables.
2 tbsp cream cheese, 1 tbsp marmite
Thinly slice your potatoes using a knife or a mandolin for even slices, and layer them over the mushroom mixture, overlapping each potato slightly. Grate over the Pecorino cheese.
2 large baking potatoes
Place the lid on your casserole pan (or cover with foil) and bake for 40 minutes. Then, remove the lid/foil and bake for another 20 - 25 minutes until the potatoes are golden and crispy. Grate over the pecorino cheese to finish.