Prepare the Cheese Filling:

1. In a medium bowl, mix ricotta cheese, egg, mozzarella cheese, Parmesan cheese, Italian seasoning, salt, and pepper until well combined. Set aside.

2. Make the Sauce:

1. Heat olive oil in a large skillet over medium heat.
2. Sauté the diced onion for 5-7 minutes until softened.
3. Add garlic and cook for 1 minute until fragrant.
4. Stir in marinara sauce, tomato paste, and fresh spinach. Cook until the spinach wilts and the sauce is heated through.

3. Cook the Lasagna Noodles:

1. Bring a large pot of salted water to a boil.
2. Cook lasagna noodles to al dente according to package instructions.
3. Drain and rinse with cold water, then lay flat on wax paper to prevent sticking.

4. Assemble the Lasagna:

1. Preheat your oven to 375°F (190°C).
2. Spread a heaping cup of sauce in the bottom of a 9 x 13-inch casserole dish.
3. Lay down 4 lasagna noodles, slightly overlapping if needed.
4. Spread 1/3 of the cheese mixture over the noodles.
5. Add 1½ cups of sauce over the cheese layer.
6. Repeat these layers two more times, finishing with a layer of sauce.
7. Top with the remaining mozzarella cheese.

5. Bake the Lasagna:

1. Cover the dish with aluminum foil sprayed with nonstick cooking spray to prevent sticking.
2. Bake for 35 minutes, then remove the foil and bake for an additional 10 minutes until the cheese is golden and bubbly.

6. Rest and Serve:

1. Let the lasagna rest for 15 minutes to set the layers.
2. Garnish with chopped parsley and serve warm.