Cook the beef: In a skillet over medium heat, brown the ground beef with onion, garlic, and green pepper (if using). Drain excess fat.
Build the base: Transfer beef mixture to the slow cooker. Add pasta sauce, tomato sauce, diced tomatoes, water, Italian seasoning, and sugar. Stir to combine.
Slow cook: Cover and cook on low for 4–6 hours or high for 2–3 hours.
Add pasta: Stir in elbow macaroni during the last 30–40 minutes of cooking. Cover and cook until pasta is tender but not mushy.
Serve: Garnish with fresh parsley, parmesan cheese, or red pepper flakes. Serve warm with bread or salad.