Peel and halve the garlic. Rub the garlic clove into an ovenproof flat dish and grease well. Peel and wash the potatoes and parsnips and slice or cut them into fine slices. Layer alternately in the mould, seasoning each layer with salt and pepper.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Rub Gruyère directly over the gratin. Mix cream and milk. Season with salt, pepper and nutmeg and pour over evenly.
Mix the hazelnuts and panko and sprinkle on the gratin. Spread 3 tablespoons of butter in small pieces on the gratin and bake in the hot oven on the lowest shelf for about 50 minutes. If necessary, cover with aluminium foil about 10 minutes before the end of the baking time.