2) Melt the butter in a very large saute pan and cook the onions over low heat for 20 mins, or until tender but not browned.
3) Add the courgettes and cook, covered, for 10 mins, or until tender. Add the salt, pepper, and nutmeg and cook uncovered for 5 more mins. Stir in the flour. Add the hot milk and cook over low heat for a few minutes, until it makes a sauce.
4) Pour the mixture into an 16x20cm baking dish.
5) Combine the bread crumbs and gruyere and sprinkle on top of the courgette mixture. Dot with 15g butter cut into small bits and bake for 20 mins, or until bubbly and browned.