Heat oven to 350°F. Spray 1 1/2-quart casserole with cooking spray. Cook spinach as directed on package; drain in colander or large strainer. Press to remove excess moisture. Set aside.
Meanwhile, in 2-quart saucepan, melt butter. Add onion; cook 2 to 3 minutes, stirring frequently, until tender. Stir in flour. Gradually stir in half-and-half. Stir in gnocchi; heat to boiling. Remove from heat. Stir in cheeses and nutmeg.
Spoon about 2 cups of the gnocchi mixture into casserole. Top with spinach, then remaining gnocchi mixture. Top with tomato slices; brush with olive oil.
Bake about 30 minutes or until bubbly and browned on top and gnocchi is tender.