Pre-heat the oven (230ºC thermostat 8). Slice the onions and spread them in a plate. Peel the potatoes and slice them thinly.
Put a first layer of potatoes in the plate. Cover the potatoes with half of the crème fraîche (the potatoes must be fully covered, if needed add some single cream).
Repeat step 2. Be careful to leave a little space on top of the plate to avoid the mixture to boil over.
Season with salt, peper and nutmeg.
Reduce the heat of the oven to 210ºC thermostat 7 before putting the plate in for 90min. If the top starts to burn, cover it with aluminium foil.