1. Combine tomatoes, garlic, oregano and 1 tsp. salt in a large bowl. In a separate bowl, stir together ricotta, Parmesan and 3/4 of mozzarella.
2. Spoon 1/4 of tomato mixture into bottom of a slow cooker. Top with a layer of noodles, breaking to fit, if necessary. Sprinkle 1/3 of spinach over noodles. Spoon 1/3 of ricotta mixture over spinach. Top with 1/4 of tomato mixture. Repeat with two more layers of noodles, spinach, ricotta mixture and tomatoes. Top with a layer of noodles and remaining mozzarella.
3. Cook on low heat until noodles are soft and lasagna is warmed through, 2 to 3 hours.