Preheat the oven to 400 degrees F.

Melt the butter in a very large sauté pan and cook the onions over low heat for 20 minutes, or until tender but not browned.  As the onions are cooking, stir them occasionally and work on prepping the other ingredients.

Add the zucchini and cook, covered, for 8-10 minutes, or until tender. Add the salt, pepper, nutmeg, and garlic and cook uncovered for 5 more minutes, stirring often.

Whisk flour into the milk and add to pan, turning up heat slightly just until milk heats and starts bubbling.  Then cook over low heat for a few minutes, until it thickens and makes a sauce. Pour the zucchini mixture, and all the sauce into an 8 by 10-inch baking dish (or something close to that).

Combine the bread crumbs and Gruyere and sprinkle on top of the zucchini mixture. Drizzle a little olive oil over the top (place your finger over the opening of the bottle to make it easier to just drizzle a little) and bake for 20 minutes, or until bubbly and browned.  Cool for a few minutes before serving.