Clean and wash Brussels sprouts. Cut crosswise on the stalk with a knife and cover in boiling salted water and cook for about 15 minutes.
In the meantime, wash the fillets and pat dry well. Heat the oil in a pan and fry the fillets for about 4 minutes on each side until golden brown.
Peel and dice the onions. Dice the bacon as well. Clean or drain the mushrooms. Wash the herbs and chop finely except for a little parsley for garnish.
Season the fillets and remove them. Leave the bacon in the frying fat until crispy. Sauté the onions and mushrooms in it. Season to taste.
Drain the Brussels sprouts and pour them into an ovenproof dish. Mix the herbs and mushrooms with the bacon with the Brussels sprouts. Put the fillets on top. Roughly grate the cheese and sprinkle over it together with the breadcrumbs.
Bake in a preheated oven (top / bottom heat: 200 ° C / convection: 175 ° C / gas: level 3) for about 15 minutes.