Preparation: Clean the mushrooms and wash them well. Put the olive oil in a frying pan and when it starts to sizzle, add the mushrooms, leave them for 2-3 minutes and then add the onions, salt and pepper. Five minutes later, add the tomatoes and cook the food at normal heat for 20-25 minutes. In the mean while, carve the chestnuts and either boil them or bake them in the oven for 20 minutes. Peel them and add them in the casserole 10 minutes before the food is ready.