Heat the olive oil in a skillet over medium-high heat. Add the sirloin strips and sauté until browned on all sides. Set aside.
In a small bowl, dissolve the beef bouillon cube in hot water. Whisk in the cornstarch until fully combined and smooth.
Pour the bouillon mixture into the crockpot. Add the soy sauce, brown sugar, Worcestershire sauce, diced tomatoes, and black pepper. Stir to combine.
Add the chopped onions and browned beef to the crockpot. Stir everything together so the ingredients are well mixed.
Cover and cook on high for 3 hours or low for 6–7 hours.
About 1 hour before serving, add the sliced bell peppers to the crockpot. Continue cooking on the same heat setting until the peppers are tender.
Serve hot over rice, noodles, or mashed potatoes.
Enjoy!