In a large skillet, heat 1 teaspoon olive oil over medium heat. Add minced garlic and sauté 1-2 minutes until fragrant but not browned. Add chopped fresh spinach and cook until wilted, about 3-4 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine ricotta cheese, shredded mozzarella, grated Parmesan, egg, Italian seasoning, salt, and pepper. Add the cooled spinach and garlic mixture and stir well until the filling is uniform and creamy.
Preheat oven to 375°F (190°C). Spread 1 cup marinara sauce evenly across the bottom of a 9x13-inch baking dish.
Using a small spoon or pastry bag, carefully fill each cannelloni tube with the spinach ricotta mixture. Place filled tubes in the baking dish over the marinara sauce.
Pour the remaining 1 cup marinara sauce over the stuffed cannelloni tubes. Sprinkle extra mozzarella and Parmesan cheese on top for a golden crust.
Cover the baking dish loosely with aluminum foil and bake for 25 minutes. Remove foil and bake an additional 10-15 minutes until bubbly and lightly browned.
Allow cannelloni to rest for 5 minutes before serving. Serve hot with a simple green salad or crusty bread.