1. Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a deep 9x13-inch baking dish.

2. Bring a large pot of lightly salted water to a boil. Cook lasagna noodles in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain.

3. Cook sausage, onions, 1/3 cup parsley, and garlic in a large skillet over medium heat until sausage is browned and crumbly, 5 to 7 minutes; drain fat. Stir in tomato paste, water, tomato sauce, Italian seasoning, oregano, and basil until well combined. Cover and simmer for 5 minutes, stirring occasionally.

4. Meanwhile, mix ricotta, spinach, Parmesan, remaining 1/3 cup parsley, eggs, garlic salt, and pepper together in a large bowl until well combined.

5. Spread 2 cups sauce in the bottom of the prepared baking dish. Layer with four noodles, 1/2 of the ricotta mixture, 1/3 of the remaining sauce, and 1 cup mozzarella. Repeat layers once more, then finish with remaining noodles, sauce, and mozzarella. Cover tightly with aluminum foil.

6. Bake in the preheated oven for 45 minutes. Remove the foil and continue to bake until cheese is bubbly and starting to brown, about 10 more minutes. Let cool for 10 minutes before serving.