Preheat the oven to 200ºC, gas mark 6. Heat the oil in a wide ovenproof sauté pan, frying pan or shallow casserole. Set the Cheddar melts aside. With the heat high, add the steaks. Brown the meat and break up with a spatula for 5-7 minutes, until deep golden. Spoon out the excess fat, leaving about 1 tbsp.
Add the mushrooms and fry, over a high heat, for 5 minutes until they are deep golden and dry, and the beef is cooked thoroughly with no pink meat. Season generously with black pepper.
Add the thyme and garlic, sizzle for 1 minute, then stir in the flour. Stir in the wine until reduced then add the stock and lentils. Simmer, stirring, until thickened.
Arrange the potato slices over the top of the meat, overlapping slightly. Pull the cheese melts into small chunks and scatter here and there. Bake towards the top of the oven for 20 minutes, until the potatoes are crisp and hot, the cheese is golden and the sauce reduced a little. Finish with a few thyme leaves and serve with green vegetables, if liked.
