Starting on one long edge, use a sharp knife to butterfly the flank steak, cutting it open parallel to the cutting board, so it can open like a book with two thin layers. Stretch the flank steak out and cover with plastic wrap. Use a meat tenderizer (or rolling pin) to pound the thickest sections creating one even thin piece of flank steak, double the length of the original. Remove the plastic and sprinkle Old El Paso Fajita Seasoning over both sides of the steak. Then cut the flank steak into twelve segments, 2.5 X 5 inches long.
Seed the bell peppers and slice into thin strips. Trim the ends of the asparagus so that the tops are about the same length as the bell peppers. Discard the ends. Peel the onion and cut into thin wedges. Heat a large skillet over high heat. Once hot, work in batches to blister the peppers, asparagus, and onions in a dry pan. Leave them in only 1-2 minutes per batch, to create char marks. Then remove.
Lower the heat to medium-high. Lay several pepper strips, asparagus spears, and onion wedges across each flank steak strip, creating a cross pattern. Wrap the ends of the flank steak tightly around the veggies and secure the ends with a toothpick.
Add 1 tablespoon oil to the pan. Once hot, add 6 rolls to the skillet. Sear 1-2 minutes per side, rotating to sear all sides. Cook the rolls approximately 5-6 minutes total, for medium-rare steak. Remove the rolls and repeat with the remaining oil and rolls. Serve warm drizzled with Old El Paso Taco Sauce.
NOTE: These little roll-ups make great lunch box leftovers!