Brown the Italian sausage in a large skillet over medium heat. Add the diced onion and minced garlic and cook until the onion softens.
Stir in the marinara sauce and Italian seasoning. Season with salt and pepper and let the mixture simmer for a few minutes.
In a slow cooker, spread a thin layer of the sausage sauce on the bottom. Add a layer of broken lasagna noodles.
Spread half the ricotta cheese over the noodles, then add half the spinach and a third of the mozzarella.
Repeat the layers with more sauce, noodles, ricotta, spinach, and mozzarella.
Finish with the remaining sauce and the rest of the mozzarella and Parmesan cheese.
Cover and cook on low for 4 hours until the noodles are tender.