Stir the ricotta, spinach, mushrooms, parsley, 1/2 teaspoon salt and a few grinds of pepper in a large bowl. In a medium bowl, combine the mozzarella and parmesan.
Spread about 1/2 cup marinara sauce in a thin layer in a 6-quart slow cooker. Cover with a single layer of uncooked lasagna noodles (4 to 6 noodles), breaking them as needed to fit. Spread half of the ricotta mixture over the noodles. Top with about one-third of the remaining sauce, then sprinkle with about one-third of the mozzarella mixture. Repeat the layers (noodles, ricotta, sauce, mozzarella mixture). Top with the remaining noodles, then the remaining sauce and mozzarella mixture.
Cover and cook on low, 4 hours. Uncover and let rest 15 minutes so the excess liquid is absorbed.
Photograph by Christopher Testani
Courtesy of Food Network Magazine