Preheat the oven to 180°C (approximately 350°F).
Peel the onion and garlic and chop finely. Clean the mushrooms and cut them into thin slices. In a hot pan with oil sweat the onion with the garlic until translucent, add mushrooms, sauté briefly and mix in the herbs. Remove from heat, let cool slightly and pour into a buttered baking dish.
Mix the cream with the eggs, the cheese and creme fraiche and season with salt, pepper and a pinch of nutmeg.
Pour the mixture over the mushrooms and bake in preheated oven for about 30 minutes. Remove and serve immediately.