Heat the oven to 375°F and grease a 9-by-13-inch dish. Boil the manicotti 3 minutes short of package directions, drain and cool in one layer.
Brown the beef, drain well, add garlic for 30 seconds and cool for 10 minutes.
Mix the beef, spinach, ricotta, egg, 1/2 teaspoon salt, pepper and Parmesan. Fill each tube with the mixture.
Spread 1 cup marinara in the dish, arrange the tubes in one layer, cover with remaining sauce and mozzarella.
Cover and bake 30 minutes, then uncover and bake 8 to 10 minutes more. Rest 10 minutes before serving.