Clean, wash and drain the Brussels sprouts. Wash, peel and cut the potatoes into pieces. Cover the sprouts and potatoes and cook them in little boiling salted water for about 15 minutes.
Wash the chicken fillets and pat them dry. Heat oil in a pan and fry the fillets for about 10 minutes, turning them over. Season with salt and pepper. Peel and finely chop the onion. Heat the fat in a pan and sauté the onion.
Add flour, sweat briefly and pour in stock and milk while stirring. Bring to the boil and simmer for about 5 minutes while stirring. Cut gorgonzola into pieces and stir into the hot sauce. Season the sauce with a little salt, pepper and nutmeg.
Drain the Brussels sprouts and potatoes and place them in a greased, ovenproof dish. Cut the chicken fillets into slices, arrange them on top and cover with the gorgonzola sauce. If necessary, sprinkle with hazelnut leaves and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-30 minutes.