Clean, wash and possibly halve the mushrooms. Peel onions and cut into strips. Wash thyme, shake dry and chop coarsely.
Wash the meat, dab dry. Heat 2 tablespoons of oil in a frying pan. Fry the meat on each side for about 6 minutes. Season with salt and pepper.
Fry the mushrooms in 2 tablespoons of hot oil. Season, take out. Sauté onions in frying fat. Sprinkle with sugar, caramelize briefly. Add mushrooms. Sprinkle with flour, sweat lightly. Stir in cream, 150 ml water and stock.
Bring to the boil and simmer for 5 minutes. Season to taste with salt and pepper. Cut the meat open and put it on the bread. Spread the sauce over it and sprinkle with thyme.