Cut beef into bite-sized pieces.
Cut bell peppers, onion, and mushrooms into bite-sized pieces as well.
Heat vegetable oil in a pan and quickly sear the beef on high heat until browned on the outside.
Add the vegetables and stir-fry together.
Season with salt and pepper, then add steak sauce and ketchup, and stir-fry.
When all ingredients are evenly cooked, transfer to a plate.