This one needs (naturally) lots of onions, but that number will always depend on the size of your  dish (ideally a shallow quiche-type dish). Cut the onions in to small strips; melt the onion in butter and oil and when they’re nice and translucent, add a bit of flour, then add cream and grated gruyere; blind bake your pastry in the dish, then pour the mixture in. Put it in a medium hot oven, for ½ an hour or so until golden