Clean and wash the Brussels sprouts. Wash the meat, dab dry. Heat oil in a pan. Sauté the meat for about 5 minutes while turning it, season with salt and pepper and remove.
Melt the butter. Chop the nuts. Remove the crusts from white bread and crumble finely. Cut bacon into strips. Wash the rosemary and, except for something to garnish, pluck needles from the stalks and chop coarsely. Peel onion and chop finely.
Cook the Brussels sprouts in boiling salted water for about 15 minutes. Mix butter, nuts, bread, rosemary, bacon and onions and spread on the meat. Place on a baking tray. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes.
Drain the Brussels sprouts and let them drain. Arrange chicken filet and Brussels sprouts garnished with rosemary. Roast potatoes taste good with it.