This is a lovely recipe which they serve in bistrots over here.
Gently fry 1 kg finely sliced onions with 50g butter for 10 mins. They must stay transparent.
Add 30cl Lager and cook very gently for an hour until the liquid has evaporated.
Put this into a pastry case
Beat 3 eggs, 20cl crème fraîche, salt, pepper and a pinch of nutmeg in a bowl and pour over the onions.
Bake in a 200°C No6 oven until golden. Leave to cool a while.
It's even better if you cook the onions one day and make the tart the next.
Bon appetit!