In 5-quart Dutch oven, cook beef over high heat 6 to 8 minutes, stirring occasionally, until browned. Reduce heat to medium; add onion, garlic, oregano, salt, pepper and pepper flakes. Continue to cook 4 to 5 minutes, stirring frequently, until onion is softened. Add tomato paste; cook and stir 3 minutes. Stir in crushed tomatoes and broth; heat to boiling. Reduce heat to low; simmer uncovered 20 minutes, stirring occasionally. Remove from heat. Serve over pasta, if desired.