Pre-heat oven to 400 degrees. In a large saute pan, melt butter and onions over low heat for about 20 minutes until tender but not browned. Add zucchini and cook, covered for another 10 minutes or so, until tender. Add salt, pepper, nutmeg and cook uncovered for a few more minutes to evaporate liquid.

Add flour and stir to coat. Add hot milk and cook over low heat for a few minutes until it gets a little thicker. Pour mixture into a baking dish.

In a separate bowl, combine bread crumbs and Gruyere cheese and sprinkle on top of zucchini mixture. Top with small dots of butter. Bake for 20 minutes or until brown and bubbly.