Heat olive oil in a large skillet over medium heat.
Season sirloin roast with salt, pepper, garlic powder and barbecue seasoning.
Brown roast on all sides in skillet. Transfer to slow cooker.
Add can of diced tomatoes to the slow cooker.
Add beef broth, soy sauce, Worcestershire sauce, and sugar.
Mix water and corn starch together and pour over ingredients in the slow cooker.
Add minced garlic.
Top with sliced onions.
Cover and cook on high for 2 hours or on low for 4 hours.
Add sliced red bell peppers and continue to cook either on high for another 2 hours or on low for another 4 hours.
Shred or slice beef and return the beef to the slow cooker; stir to combine with pepper/onion mixture.