Place a large skillet over medium-high heat and add oil.
Once the oil is simmering, add the onion and garlic, using a wooden spoon, stir and cook for one minute. Add the beef and the ground sausage.
Let the beef brown a bit and start breaking it with a wooden spoon. Cook until no longer pink. Drain any accumulated grease.
Add tomato sauce, crushed tomatoes, Italian seasoning, salt, and pepper to taste. Stir and let simmer on low for about 30 minutes.
Add the spinach, stir and cook until wilted. Remove from heat and set aside.
Prepare Cheese Mixture:
In a large bowl combine: ricotta cheese, parmesan cheese, 2 cups of mozzarella cheese, beaten egg, Italian seasoning, salt, and pepper to taste. Stir until fully combined.
Lightly grease a 9×13 casserole dish with olive oil.
Spread about 1/2 cup of the meat mixture on the bottom and add one layer of cooked lasagna noodles to cover the bottom of the pan.
Spread 1/2 of the cheese mixture, then top with 1/2 of the meat mixture. Spread about 1 cup of marinara sauce and sprinkle about 1/2 cup of the mozzarella cheese (from the remaining cup of mozzarella cheese).
Add another layer of lasagna noodles, top with the all remaining cheese mixture, followed by the beef mixture, a cup of marinara sauce, and the remaining 1/2 cup of mozzarella cheese.
Add the final layer of lasagna noodles, top with 1 cup of marinara sauce and 1 1/2 cups of shredded mozzarella cheese.
Preheat oven to 375 degrees F. Bake for 40 minutes.
If needed, broil at the end to make the cheese golden brown and bubbly.
Remove from oven and let the lasagna rest for 15-30 minutes before slicing.