Bring a large pot of salted water to a boil and cook the manicotti pasta until al dente (about 10-12 minutes). Drain and rinse with cold water. While the pasta cooks, heat oil in a skillet.
In a large bowl, combine the onion, garlic, ricotta, shredded mozzarella, 1/2 cup of the grated Parmesan, egg, Italian seasoning, spinach, salt, and pepper until well blended.
Spread 1/2 cup of the spaghetti sauce into the bottom of a baking dish. Using a spoon or piping bag, fill each manicotti shell with the cheese mixture and arrange them over the sauce.