Begin by bringing a large pot of salted water to a boil. Add the lasagna sheets and cook until al dente, about 8-10 minutes. Drain and rinse with cold water.

In a skillet, heat olive oil over medium heat. Add chopped onion and sausage, breaking it apart. Cook until browned, about 5-7 minutes. Stir in minced garlic, salt, pepper, and herbs, cooking for an additional minute. Add tomato sauce and simmer for 10-15 minutes.

Preheat oven to 350°F (175°C). Gather your baking dish and measuring cups for assembly.

In a mixing bowl, combine ricotta cheese, beaten egg, spinach, mozzarella, and parmesan. Mix until well incorporated.

Spread a thin layer of meat sauce in the baking dish. Use about 1 cup of sauce.

On each lasagna sheet, spread the ricotta filling, add a spoonful of meat sauce, and roll tightly. Place seam-side down in the baking 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.

Let cool for 5-10 minutes before serving.