Cut the sirloin into bite-sized pieces.
Cut the onion, bell pepper, and garlic into pieces of a suitable size for eating.
Heat olive oil in a pan and sauté the sirloin.
When the sirloin is about halfway cooked, add the vegetables and stir-fry together.
Season with tomato sauce, black pepper, and salt.
Once all the ingredients are evenly cooked, transfer to a plate.