Dice the onion and carrot, and slice the mushrooms. Gently fry the vegetables in the oil until just cooked. Add chopped or minced meat with seasoning to taste. Stir in the flour then the stock, bring to the boil and cook for a few minutes. Press just under half the potato on to an eight-inch greased overproof plate, rather like pastry. Pile the meat mixture into the centre. Press the remaining potato flat over the top or roll out on a floured surface and lift on. Press the edges well together, brush the top with a little melted butter and sprinkle the cheese on top. Bake at 400°F, 200°C, gas mark 6 for thirty minutes.