Prepare the Beef: Slice the sirloin steak thinly against the grain for tenderness.
Sear (Optional): Heat olive oil in a skillet and brown the beef for 2–3 minutes per side to lock in flavor.
Layer Ingredients: Place the onions and bell peppers at the bottom of your slow cooker, then add the browned beef on top.
Make the Sauce: In a bowl, whisk together soy sauce, beef broth, brown sugar, honey, Worcestershire sauce, garlic, and black pepper. Pour over the meat and vegetables.
Cook: Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until beef is tender and peppers are soft.
Thicken (Optional): Mix cornstarch with a small amount of water and stir into the sauce during the final 20 minutes of cooking.
Serve: Garnish with parsley or green onions. Serve warm over rice, noodles, or mashed potatoes.