Slow Cooker Lasagna Stew

1 pound uncooked lean ground beef (with 7% fat)
1 small onion(s), chopped
1 medium garlic clove(s), minced
28 oz canned crushed tomatoes
15 oz canned tomato sauce
1 tsp table salt
1 tsp dried oregano
1/2 tsp dried basil
1/4 tsp crushed red pepper flakes, or to taste
1 cup part-skim ricotta cheese
1 1/2 cup part-skim mozzarella cheese, shredded, divided
6 items dry lasagna noodles, no-cook
1/2 cup shredded Parmesan cheese, strong-flavored like Romano or Parmigiano Reggiano
Heat a large nonstick skillet over medium-high heat. Add beef, onion and garlic, and cook, stirring frequently and breaking up meat with a wooden spoon as it cooks, about 5 to 7 minutes. Stir in crushed tomatoes, tomato sauce, salt, oregano, basil and crushed red pepper flakes; simmer 5 minutes to allow flavors to blend. Meanwhile, in a medium bowl, stir together ricotta cheese and 1 cup of mozzarella cheese. Spoon 1/3 of beef mixture into a 5-quart slow cooker. Break 3 lasagna sheets in half and arrange over beef mixture; top with half of ricotta mixture. Repeat with another layer and finish with remaining 1/3 of beef mixture. Cover slow cooker and cook on low setting for 4 to 6 hours. Remove cover; turn off heat and season to taste, if desired. In a small bowl, combine remaining 1/2 cup of mozzarella cheese and Parmesan cheese; sprinkle over beef mixture. Cover and set aside until cheese melts and lasagna firms up, about 10 minutes