Cut the sirloin steak into bite-sized cubes and pat dry with a paper towel. Season with salt and pepper.

Prepare the vegetables by slicing the onion and bell peppers.

Mix the sauce ingredients: steak sauce, ketchup, corn syrup, minced garlic, oyster sauce, and mustard sauce. Optionally, add a small amount of butter for enhanced flavor.

Heat oil in a pan and sauté the onion until slightly softened. Add the beef and cook until lightly browned.

Add the bell peppers to the pan and stir-fry briefly. Keep them crisp by adding them last.

Pour the prepared sauce into the pan and stir-fry everything until well combined and heated through.