Boil salted water in a large pot, add lasagna sheets, and cook until al dente, about 8-10 minutes. Drain and cool on a clean surface.

In a skillet, heat olive oil, sauté chopped onion for 3-4 minutes, then add crumbled Italian sausage and cook until browned, about 5-7 minutes. Stir in minced garlic and herbs, add tomato sauce, and simmer for 10-15 minutes.

Preheat oven to 350°F (175°C). In a mixing bowl, combine ricotta cheese, egg, ¼ cup grated Parmesan, 1 cup shredded mozzarella, and chopped spinach. Stir until creamy.

Spread ¼ cup meat sauce on a baking dish. Take a lasagna sheet, spoon 2 tablespoons ricotta mixture and 2 tablespoons meat sauce, roll them tightly and place seam-side down in the dish.

Pour remaining meat sauce over the rolls and sprinkle with remaining mozzarella and Parmesan. Cover with foil and bake for 40 minutes.

Remove foil and bake for an additional 5 minutes until cheese is golden and bubbly. Let cool for 10 minutes before serving.