Sprinkle beef tips with Cajun seasoning.
Cook beef in hot oil in large skillet over medium-high heat 10 minutes or until browned. Add bell pepper, onion, and garlic; sauté 3 minutes.
Stir in broth and next 6 ingredients. Bring to a boil; reduce heat, cover, and simmer 1 hour or until meat is tender.
Cook frozen mashed potatoes according to package directions.
Stir together cornstarch and water until smooth; stir into meat mixture. Bring to a boil; cook, stirring constantly, 2 minutes or until thickened. Serve over mashed potatoes.