Sear the Steak: Heat the olive oil in a large skillet over medium-high heat. Add the sliced sirloin steak and sauté for 3 to 5 minutes, just until the outside is nicely browned. (Do not cook completely through).

Make the Slurry: In a small bowl or measuring cup, dissolve the beef bouillon cube into the ⅔ cup of hot water. Whisk the cornstarch into the bouillon mixture until completely smooth and free of lumps. Pour this mixture into the bottom of your crockpot.

Add Flavorings: To the crockpot, add the minced garlic, soy sauce, brown sugar, undrained diced tomatoes, Worcestershire sauce, and black pepper. Stir lightly to combine the liquids.

Combine Meat and Onions: Add the chopped onions and the browned steak (along with any delicious juices left in the skillet) into the crockpot. Toss to ensure the meat is well-coated in the sauce.

Slow Cook: Secure the lid and cook on HIGH for 3 hours, or on LOW for 6 to 7 hours, until the steak is incredibly tender.

Add Peppers: Exactly one hour before serving, stir the sliced bell peppers into the crockpot. Cover and continue cooking for the remaining 60 minutes. This timing ensures the peppers are cooked through but still slightly crisp.

Serve: Taste the gravy and adjust salt and pepper if necessary. Serve hot over steamed rice, noodles, or cauliflower rice.