In a small bowl, mix together seasoning mix. Set aside.
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 mallet (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 seasoning mix over both sides of the steak. Then cut the flank steak into twelve segments, approximately 2.5 X 5 inches long. NOTE: If it is easier to work with, you can cut into strips, and butterfly those then pound so you are working with smaller pieces.
Seed the bell peppers and slice into thin strips.
Trim the ends of the asparagus so the tops are about the same length as the bell peppers. Discard the ends.
Peel the onion and cut into thin wedges.
Put a cast iron skillet over high heat, and heat without adding any liquid or oil. Once hot, add the . peppers, asparagus and onions into the pan, and give them space. Work in batches to blister them in a dry pan without crowding them. 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.
Wrap the flank steak tightly around the veggies and secure the ends with a toothpick.
Add 1 tablespoon oil to the pan and heat.
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.
Top with hot sauce if desired.