Prepare the Ricotta Mixture: In a spacious bowl, combine the ricotta cheese, squeezed spinach, Parmesan, basil, garlic, oregano, onion powder, salt, and a generous amount of black pepper. Stir until evenly blended.
Grease the Slow Cooker: Lightly coat the interior of a 6- to 7-quart slow cooker with olive oil. Pour approximately 1 cup of marinara sauce into the base of the slow cooker and spread it evenly to cover the bottom.
Layer the Ingredients: Begin by layering three lasagna noodles over the sauce. It is acceptable if the noodles overlap slightly or if they do not cover every inch of the surface. Spread ¾ cup of sauce over the noodles and evenly distribute half of the ricotta mixture on top. Use a fork to spread the mixture into a smooth, even layer, then sprinkle one-third of the shredded mozzarella cheese over the ricotta. Repeat this process with another layer of three noodles, followed by ¾ cup of sauce, the remaining ricotta mixture, and another half of the remaining mozzarella.
Final Layer: For the final layer, place the remaining three lasagna noodles on top and spread them with the remaining marinara sauce. Reserve the remaining mozzarella for the topping.
Slow-Cook the Lasagna: Cover the slow cooker and cook on low heat for 3.5 to 4 hours, or until the internal temperature reaches 140°F and the noodles are tender.
Finish and Rest: Once the lasagna is cooked, turn off the slow cooker. Sprinkle the remaining mozzarella over the top of the lasagna, cover, and allow it to rest with the lid on for 20 to 30 minutes. This step ensures the lasagna holds together when sliced. Before serving, garnish with torn basil leaves to taste. Carefully slice and serve with a spatula for a neat presentation.