Soak porcini mushrooms in 1/8 litre lukewarm water. Peel and finely dice the onion. Dice bacon and fry it crisply in a pan. Add onions. Grate toast. Add mustard and thyme to the bacon-onion mixture.
Cut the beef fillet diagonally and fill the filling into the incision. Peel and wash the potatoes. Place on a tablespoon and cut the potatoes finely. Spread with soft fat and sprinkle with salt and pepper.
Place the beef fillet and potatoes on a baking tray and cook in a preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 30 minutes. Peel, wash and cut carrots into pieces. Cook in boiling salted water for 10 minutes.
Clean oyster mushrooms and rub with kitchen paper. Heat the remaining fat in a pan and fry the mushrooms in it until golden brown. Take them out. Deglaze the frying fat with the porcini mushrooms and the mushroom water.
Add cream and let it boil down a little. Stir in sauce thickener, bring to the boil and season with salt and pepper. Drain the carrots and add to the sauce with the oyster mushrooms. Remove the meat, cut it open and arrange it with potatoes and vegetables on a preheated plate.
Garnish with thyme.