1. Before beginning, preheat the oven at 350 degrees Fahrenheit (180 degrees Celsius). Grease a 9×13-inch baking dish lightly.
2. Bring a big saucepan of lightly salted water to a boil to begin with. Cook for 8 to 10 minutes, or until lasagna noodles are al dente; afterward, drain.
3. Brown the sausage with the onions, garlic, and 1/2 of the parsley in a pan over medium heat; drain excess grease. Mix the tomato paste, tomato sauce, water, Italian seasoning, oregano, and basil until everything is thoroughly combined. Cook for 5 minutes, covered, stirring periodically.
4. Combine the remaining parsley and ricotta, spinach, Parmesan, eggs, garlic salt, and pepper in a mixing dish and stir thoroughly.
5. Two cups of sauce should be spread in the bottom of the baking dish. Layer 4 noodles, ½ of the cheese mixture, ⅓ of the remaining sauce, and 1 cup of mozzarella on top. Repeat the layering process, ending with noodles, sauce, and mozzarella cheese as the final layer. The pan should be covered with aluminum foil.
6. Bake for 45 minutes in the preheated oven. Remove the foil and bake for another 10 minutes, or until the cheese is bubbling and beginning to brown. Allow for a 10-minute cooling period before serving.

The total cooking duration for this recipe is 1 hour and 50 minutes, with 40 minutes of preparation time, 1 hour cooking time, and an additional ten minutes resting time. Do you need more ideas on how to make lasagna? Then you should check out this video recipe.