Wash and deseed bell peppers, then slice them into 1/4" (0.5 cm) thick long stripes. Set aside.
Peel and slice red onion. Set aside.
Once skillet is hot, add a splash of oil. When the oil is hot, add stir-fry strips in 2-3 batches. Make sure the strips don't touch each other.
Salt and pepper each beef stir-fry batch generously in the pan. Cook for about 1 minute per side, then set aside on a plate and cover to keep warm.
Once all the beef is cooked and set aside, add sliced onions and bell peppers to the remaining meat juice. Season with cumin and chili powder, then stir-fry until desired consistency.
Transfer the veggies and beef stir-fry strips to a plate and serve with sliced avocado, a drizzle of lemon juice, and a sprinkle of fresh cilantro.
This recipe is for 4 servings of beef fajitas.Take the beef stir-fry strips out of the refrigerator 30 minutes before cooking them. By letting them come to room temperature, they won't lower the temperature of the pan, which will give you better frying results.Also, cook the beef in batches, ensuring none of the strips are touching each other. This makes sure each piece is evenly fried.Make sure the pan is smoking hot before adding the beef strips.When cooking bell peppers, cook for about 5 minutes if you like a little crunch or longer if you like them a bit softer.Leftovers can be stored in an airtight container in the refrigerator for 3-4 days, but they are best stored without the avocado, as sliced avocado will quickly oxidize and turn brown and mushy.If meal-prepping, you can cook the beef stir-fry strips ahead of time and store in the refrigerator. Then, when you're ready to eat, cut and fry the veggies and slice the avocado before serving.