For the meat mixture: In a large skillet over medium-high heat, add the olive oil and onion and saute for 5 minutes. Add the ground beef, sausage and garlic and cook until brown, 7 to 9 minutes. Add the tomatoes with their juice, tomato paste, basil, parsley, oregano, salt and some pepper and stir. Set aside.
For the cheese mixture: In a medium bowl, combine the ricotta, Parmesan, Romano, basil, parsley, salt, pepper and eggs and stir together well.
For the lasagna: Spray a 6-quart slow cooker with cooking spray.
Spoon in one-quarter of the meat mixture on the bottom and top with 5 lasagna noodles, broken to fit. Add a third of the cheese mixture, then sprinkle with 1/2 cup mozzarella. Repeat this layering twice, starting with the meat mixture and ending with the mozzarella, for 3 layers total.
For the fourth layer, top with the remaining meat mixture and remaining 1/2 cup mozzarella. Cover and cook on low for 4 hours. Turn off the heat and allow the lasagna to sit 30 minutes before serving. Sprinkle with chopped parsley.