Cut the leeks into rounds and wash them. Peel the onions and dice finely. Cut the bacon into strips. Heat the oil in a frying pan. When it is hot, add the leaks and the onions and fry for 3 minutes over a hot flame, stirring constantly.
Next, lower the heat, add a glass of water and season with salt and pepper. Leave to cook gently for 15 minutes.
Meanwhile, beat together the cream and egg. Season with salt and pepper.
Preheat the oven to Gas Mark 7, 210°C.
Grease the gratin dish. Add the bacon to the frying pan, cook for a further one or two minutes and then pour the mixture into the gratin dish.
Pour the cream and egg mixture over the top and then cover with a layer of grated cheese. Bake in the oven for 20 minutes. Serve straight from the oven, accompanied by red meat or as a main dish alongside some baby leaf salad.
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