In a large skillet, heat the olive oil over medium-high heat. Add the strips of sirloin and sauté until browned on all sides. Do not cook through; this step is to develop flavor.
Prepare the broth base
In a measuring cup, dissolve the beef bouillon cube in the hot water. Whisk in the cornstarch until fully combined, then pour the mixture into the crockpot.
Add seasonings and sauce
Stir in the Worcestershire sauce, soy sauce, and brown sugar. Add the diced tomatoes along with a pinch of salt and black pepper to taste.
Combine beef and onion
Transfer the browned steak strips and sliced onion to the crockpot. Stir gently to coat the ingredients evenly in the sauce.
Slow cook
Cover and cook on low for 6 to 7 hours, or on high for approximately 3 hours, until the steak is tender.
Add peppers
One hour before the end of cooking time, stir in the sliced red and green bell peppers. Continue cooking until the peppers are tender yet still slightly crisp.
Serve
Garnish with green onions if desired. Serve the pepper steak over rice, noodles, or mashed potatoes, accompanied by a fresh salad.
Notes
To prepare this recipe gluten-free, replace soy sauce with certified gluten-free tamari or coconut aminos. Ensure that the beef bouillon and Worcestershire sauce used are also labeled gluten-free.