Melt butter in skillet over low heat.
Add flour; stir for 3 minutes.
Add milk, nutmeg, salt, pepper and sugar.
Stir until thickened.
Add spinach-onion mixture.
Arrange zucchini shells in buttered, shallow, oven-proof dish.
Divide filling among the shells.
Sprinkle each with 1 tablespoon Parmesan and bake in a preheated 350Â° oven for 30 minutes.