Preheat oven to 375°F (190°C).

Cook manicotti shells according to package directions until al dente. Drain and let cool.

Heat olive oil in a skillet over medium heat. Add ground beef and cook until browned.

Add garlic and spinach; cook until spinach wilts. Remove from heat and cool slightly.

In a large bowl, combine beef mixture, ricotta, 1 cup mozzarella, Parmesan, egg, Italian seasoning, salt, and pepper.

Spread 1 cup marinara sauce in the bottom of a baking dish.

Fill each manicotti shell with the beef and spinach mixture and place in the dish.

Pour remaining sauce over stuffed shells and sprinkle with remaining mozzarella.

Cover with foil and bake for 30 minutes. Uncover and bake an additional 15 minutes until bubbly and golden.

Rest for 5–10 minutes before serving.