Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius).
Drizzle the olive oil on a large sheet pan.
Add the sliced onions and sliced bell peppers to the sheet pan, along with 1 1/2 tablespoons of the taco seasoning.
Using your hands or a pair of tongs, toss everything together until the veggies are well coated in the seasonings and the oil.
Bake at 400 degrees Fahrenheit (200 degrees Celsius) for about 8-10 minutes, or until the veggies are becoming soft.
Slice your steak against the grain in 1/4 inch thick slices. Add the rest of the taco seasoning, lime zest and juice to the steak slices and mix well.
Add the steak slices on top of the veggies and return to the oven for 6-8 more minutes.
Serve in tortillas with lettuce, tomatoes, cheese, sour cream and avocado, as desired (use the time while the fajita filling is baking to prep these ingredients).