Preheat the oven to 200C/400F/Gas Mark 6. Melt the butter in a medium-sized pan, add the onion and cook gently until soft but not coloured.
Add the mushrooms and thyme and continue to cook until all the excess liquid has evaporated and the mixture is quite dry. Season to taste with salt, pepper and nutmeg.
Spoon the mushroom mixture into six lightly buttered ramekin dishes. Break in the eggs, season them lightly, then spoon over the cream and sprinkle with the GruyÃ©re cheese.
Put the ramekins into a shallow roasting tin and pour some hot water around them so that it comes about half way up the sides of the dishes. Bake for 15 minutes until the eggs are set but the yolks are still runny and the cheese is lightly golden.
Serve with a mixture of seasoned baby salad leaves and herbs, dressed in lemon olive oil or extra virgin olive oil.