1. Preheat the oven to 160 °C (5-6).
2. Slice the onions and cook them gently for an hour in a little olive oil and butter in mini square Pyrex® dishes.
3. Peel and thinly slice the potatoes. In the mini casseroles, place several layers of potatoes and onions.
4. Mix the eggs, cream and nutmeg together. Season to taste and delicately pour this mixture on the potatoes.
5. Cook for 35 minutes. Cover and serve.