This simple five-ingredient stir-fry recipe is a perfect choice when you have some leftover roast beef on hand. All you need is some frozen mixed vegetables and some stir fry sauce and you'll have dinner on the table in less than 15 minutes. There are many stir-fry sauces on the market; choose the one you like best. And you can use other vegetables in this recipe if you'd like; choose a combination of broccoli, carrots, and snap peas or any other type of vegetable. If you make a change, be sure to write it down so you can reproduce the recipe in the future.