Wash the mushrooms, break out the stems and chop finely. Peel onion and chop finely. Heat oil in a pan and fry the onion in it. Add the mushroom stems and fry. Season with salt and pepper. Wash thyme, dab dry and pluck the leaves from the stems. Mix sausage meat with mushroom stems and mustard.
Stir in the thyme, except for a little bit for sprinkling. Season mushroom heads with salt and pepper, fill with the sausage meat and place in an ovenproof dish. Coarsely grate the cheese and spread on the mushrooms. Wash and clean the tomatoes, dice them coarsely and distribute them in the dish. Pour on the stock. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for about 20 minutes. Sprinkle with the remaining thyme before serving