Heat a large skillet over medium-high heat and add 2 tablespoons olive oil.
Add the red, green, and yellow bell pepper strips, onion strips, and halved asparagus to the skillet.
Sprinkle 1 tablespoon of the fajita seasoning over the vegetables and sauté until almost tender, about 4–5 minutes; remove from heat and set aside.
Lay the flank steak pieces flat and pound each to about 1/4 inch thickness; slice each piece in half lengthwise if not already done so.
Divide the cooked vegetables evenly down the center of each steak piece, then roll each steak tightly around the filling and secure with a toothpick.
Return the skillet to medium-high heat. Season the outside of the steak roll ups with additional fajita seasoning to taste.
Cook the roll ups in the hot skillet, turning to brown all sides, about 2–3 minutes per side, until the steak is cooked through to your liking.
Remove toothpicks before serving and slice or serve whole.