Boil porcini mushrooms with 200 ml of boiling water. Coarsely chop the walnuts. Finely grind white bread in the universal chopper. Clean and wash the Brussels sprouts. Peel and wash potatoes. Drain mushrooms well in a sieve, collect mushroom water.
Squeeze out the mushrooms and chop very finely. For the crust mix mushrooms, bread crumbs, nuts and 75 g butter. Cover and cook potatoes and 250 g Brussels sprouts in boiling salted water for about 25 minutes. Cook the rest of the sprouts in boiling salted water for 18-20 minutes.
Wash the meat and dab dry. Heat oil in a pan, fry meat for 2 minutes on each side, season with salt and pepper. Remove from the pan, place in an ovenproof dish, spread the crust evenly on top and bake in the preheated oven (electric range: 225 °C/ convection oven: 200 °C/ gas: level 4) for 10-12 minutes on the middle shelf.
Deglaze frying fat with beef stock and mushroom water, bring to the boil, let simmer for about 6 minutes. Stir in sauce thickener and season with salt and pepper. Meanwhile bring milk and 50 g butter to the boil. Drain potato and Brussels sprouts mixture.
Add milk and mash with a potato masher, season with salt and nutmeg. Drain the Brussels sprouts. Serve the meat, some sauce, Brussels sprouts and mashed potatoes and Brussels sprouts on plates.