In a large nonstick pan, brown ground beef with onions over medium-high heat until no longer pink. Be sure to break up beef into a crumble as it cooks. Remove from heat. Pour into a colander in the sink to drain the excess fat. Add back to pan and stir in Italian seasonings, crushed red pepper and black pepper.
In a large bowl, combine ricotta cheese, spinach and egg. Mix well.
To assemble lasagna: Spread 1 cup of pasta sauce in bottom of crock-pot. Top with half of noodles, breaking and overlapping as necessary to fit. Add half of beef mixture and spread all around. Next, spread 1 cup of the pasta sauce and add ¼ cup water. Spread half of the cheese/spinach mixture over pasta sauce. Top with remaining 3 noodles. Top with remaining beef. Spread remaining cheese/spinach mixture. Finally top with 1 cup pasta sauce. Spread evenly over top.
Cover and cook on low-heat setting for 3-3½ hours or cook for 1½-2 hours on high heat until hot and lasagna noodles are soft. Once ready, top with ½ cup pasta sauce and 1 cup shredded cheese. Let stand, covered, for 10 minutes before serving.
To serve: Cut lasagna into 8 portions. Remove each slice from crock-pot using a spatula. Leftovers freeze great!