Clean the Brussels sprouts and cut in half. Cook in boiling salted water for 10 minutes, drain, and set aside.
Cook the rice for 10 minutes and set aside.
Peel and roughly dice the onion. Season the chicken with salt and pepper, then rub with mustard. Fry slowly in olive oil with the onion, bay leaves, lemon zest, and garlic.
Cover with the milk and braise gently for 15 minutes.
Preheat the oven to 180°C (top and bottom heat).
Remove the chicken from the pan and cut into large strips. Layer rice, Brussels sprouts, and chicken in a baking dish.
For the topping: remove 400 ml of the milk from the pan and whisk with the sour cream and eggs. Season with salt, pepper, and nutmeg. Pour over the dish.
Sprinkle with the almonds, butter cubes, and Parmesan.
Bake in the oven at 180°C for about 35 minutes, until golden and set.