In a large saucepan, melt butter over medium heat. Add in onion, green pepper and garlic and sauté for 10 minutes until onion is softened and translucent.
Stir in cayenne pepper, crushed red pepper flakes, salt, diced tomatoes (with their juices), cornstarch and bring to a boil.
Reduce heat to medium-low and stir in shrimp. Simmer, covered, for 6 to 10 minutes or until shrimp is cooked.
Serve over a bed of rice and top with green onions, if desired.