Add the zucchini and cook, covered, for 10 minutes, or until tender.
Add the salt, pepper, and nutmeg and cook uncovered for 5 more minutes.
Stir in the flour. Add the hot milk and cook over low heat for a few minutes, until it makes a sauce. Pour the mixture into an 8 by 10-inch baking dish.
Combine the bread crumbs and Gruyere and sprinkle on top of the zucchini mixture.
Dot with 1 tablespoon of butter cut into small bits and bake for 20 minutes, or until bubbly and browned.
Recipe by The Peony Life at http://thepeonylife.com/2015/08/summer-squash-gratin/