Cook manicotti according to package directions. Drain. Cool manicotti in a single layer on a piece of greased foil.
Meanwhile, for sauce, in a medium saucepan cook green onion and garlic in butter until tender. Stir in flour. Add milk all at once. Cook and stir until thickened and bubbly. Add Swiss cheese and broth, stirring until cheese melts.
For filling, in a medium bowl stir together egg, spinach, ricotta cheese, Parmesan cheese, and lemon peel. Use a small spoon to fill manicotti shells with filling. Arrange filled shells in a 2-quart rectangular baking dish. Pour sauce over filled shells.
Bake, covered, in a 350 degree F oven for 30 to 35 minutes or until heated through. Let stand for 10 minutes before serving.