1. In a bowl, mix together the spinach, ricotta, and ½ cup of the Parmesan. In a second bowl, mix together the marinara sauce and ½ cup water.
2. Spread ¾ cup of the marinara mixture in the bottom of a 4- to 6-quart slow cooker. Top with 2 noodles (breaking to fit), ¾ cup of the remaining marinara mixture, half the spinach mixture, and ½ cup of the mozzarella; repeat. Top with the remaining noodles, marinara mixture, mozzarella, and Parmesan.
3. Cover and cook on low until the noodles are tender, 3 ½ to 4 hours.
4. In a large bowl, whisk together the oil, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. Add the lettuce, cucumber, and onion and toss to combine. Serve with the lasagna.
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