Peel and wash the potatoes and cut them into 2-3 mm thick slices. Peel garlic and cut in half lengthwise. Rub garlic into any flat ovenproof dish and put aside.
Layer potato slices in it. Bring cream, milk, 1 bay leaf and 1 sprig of rosemary to the boil. Season with salt and nutmeg. Pour the cream mixture over the potatoes. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for 40-45 minutes.
Peel the pumpkin and remove the seeds. Cut the flesh into medium-sized cubes. Place pumpkin, garlic, remaining bay leaf and rosemary on a piece of aluminium foil. Sprinkle with sea salt and drizzle with olive oil.
Seal aluminium foil. After approx. 15 minutes, place in the oven. In the meantime wash the chicken fillets, dab dry and season with salt and pepper. Heat oil in a pan and fry the chicken fillets in it for about 8 minutes on both sides.
Remove and keep warm. Drain the cooking fat, deglaze the meat with stock and bring to the boil. Stir cornflour with 1 tablespoon of water until smooth and thicken the stock. Cut the chicken fillets open.
Cut out gratin at will. Arrange chicken breasts with gratin, pumpkin vegetables and sauce on plates.