If serving the beef stir fry with rice or noodles, get that cooking now.
Then, prepare your ingredients. Cut the beef sirloin into thin, evenly-sized strips. Each strip should measure about 2 inches long. Remove any fat or gristle as you go along.
Cut up your vegetables (see note below). Cut broccoli into similar-sized, bite-sized pieces. Thinly slice the carrots and peppers until they measure around 2 inches long. Remove the pith and seeds from the peppers. If needed, mince the garlic at this time as well. Set the vegetables aside.
Heat your choice of oil in a wok, cast iron skillet, or other heavy-based skillet over medium heat.
Once the oil is hot, carefully add the beef. Cook just until browned. Remove the beef from the skillet and set it aside for now.
If needed, add more oil to the skillet. There should be enough liquid left behind from cooking the beef. Add in all of the prepared vegetables minus the garlic. Cook until soft or preferred texture.
Add the minced garlic and cook until soft, about 2 to 4 minutes.
Add the beef back to the skillet and mix with the vegetables. Season with the soy sauce. Cook for 2 more minutes to help heat everything through. Taste and add more soy sauce if desired. Serve over cooked rice or noodles.
Notes
You can use almost any combination of vegetables for this recipe. We have also used zucchini, mushrooms, and sugar snap peas. Aim for around 3 cups of vegetables total. If using softer vegetables like zucchini and mushrooms, add those at the same time as the garlic.