Put the tuna fish on a sieve and let it drain. Wash the peppers, cut off the lid and clean. Cut the lid into cubes. Put the tomatoes briefly in boiling water, remove and rinse.
Peel the skin, quarter the tomatoes, remove the seeds and cut the flesh into fine cubes. Clean, wash, drain and slice the mushrooms. Wash parsley and thyme, dab dry (put some sprigs of thyme aside).
Pluck the thyme leaves from the stalks and chop finely together with the parsley. Put tomatoes, mushrooms, herbs, egg and grated cheese in a bowl and mix carefully. Season to taste with salt and pepper and fill into the peppers.
Put the peppers in an ovenproof dish. Mix stock and tomato ketchup. Add the diced peppers and stock to the peppers. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 30-40 minutes.
Remove the peppers and keep warm. Stir the crème fraîche into the stock and season to taste. Serve the peppers with the sauce and garnish with sprigs of thyme.