chopped frozen spinach, thawed and squeezed to remove excess moisture
cups marinara sauce
onion, thinly sliced (optional)
In a bowl, mix together the spinach, ricotta, and some of the Parmesan.
Spread some of the marinara in the bottom of a slow cooker. Top with noodles (breaking to fit), cover with some of the marinara, half the spinach mixture, and some of the mozzarella; repeat. Top with the remaining noodles, marinara, mozzarella, and Parmesan. (If desired add some onions to a layer)
Cover and cook on low until the noodles are tender, 3 ½ to 4 hours.