Heat a large nonstick skillet over medium-high heat. Add beef, onion and garlic; cook, stirring frequently, 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 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 of 1/3 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.
You can boost vegetable intake - add 2 cups of sliced mushrooms to the beef misture or a can of zucchini in tomato sauce in the meat mixture.