3 cans (14.4 ounces) crushed tomatoes
3 cloves garlic , finely chopped
2 Tbsp. dried oregano
1/2 tsp. kosher salt
1/4 tsp. red pepper flakes
1/4 tsp. freshly ground black pepper
2 containers (15 ounces each) fresh ricotta
2 cups (8 ounces) grated mozzarella
1/4 cup grated Parmesan
12 lasagna noodles (about three-quarters of a 1-pound box)
5 ounces (about 6 cups) baby spinach
In a medium bowl, combine the tomatoes, garlic, oregano, salt, red pepper and black pepper. In a separate bowl, mix the ricotta, 1 cup of the mozzarella and Parmesan.
In the bottom of a 5- to 6-quart slow cooker, spread a thin layer of the sauce. Top with 3 of the noodles (breaking to fit as necessary). Spread about 1 cup of the sauce over the noodles and layer with 2 cups of the spinach and 1 1/2 cups of the cheese mixture. Repeat twice more with the noodles, sauce, spinach and cheese mixture. Top with the remaining 3 noodles, sauce and 1 cup mozzarella.
Cook on low, covered, until the noodles are tender—especially in the center of the lasagna—about 3 to 3 1/2 hours.