In a large skillet, brown the hamburger meat and onion together. Drain any excess fat/grease.
Add the bell pepper, celery, tomato sauce and paste, Worcestershire sauce, garlic, brown sugar, mustard, and red pepper flakes. Stir.
Bring to a slow slimmer and reduce heat to low. Cover and simmer until the desired tenderness of the vegetables.
Toast the buns if desired, and scoop mixture onto buns.