In 10-inch skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring occasionally, until beef is brown; drain. Reduce heat to medium; add bell pepper; continue to cook 3 minutes. Add brown sugar, vinegar, Worcestershire sauce, salt, pepper and pepper flakes. Cook and stir 2 minutes. Add crushed tomatoes and tomato paste. Stir to combine. Reduce heat to simmering; cook 5 minutes, stirring occasionally.