Preheat the grill to medium high. If using a charcoal grill, prepare it for indirect heat.
In a large skillet, heat the oil over medium heat until shimmering. Add the peppers and onion and sauté until just tender, about 5 to 7 minutes. Add the taco seasoning and continue to cook another 2 minutes.
Lay the flank steak open like a book on a work surface. Sprinkle with salt and pepper. Arrange the peppers and onions over so that they are all lying with the grain of the meat. Leave about 2 inches at one end of the meat with no fillings on it (this helps when rolling it up). Arrange the slices of pepper jack cheese overtop of the peppers and onions.
Starting at the end that is well covered by fillings and working toward the uncovered end, roll the flank steak up tightly. Secure well with 5 or 6 pieces of kitchen twine. Sprinkle with more salt and pepper.
Place the roll on the grill over direct heat and brown on all sides, about 2 minutes per side. Then move the roll over indirect heat (if using a gas grill, turn off one element and set the roll over the this part). Place so that the seam of the roll is facing up (less cheese will leak out this way!)
Let the flank steak continue to cook over indirect heat until cooked to your liking. About 125F to 135F on an instant read thermometer for medium rare. If your cheese is melting and leaking out too much, place the roll on a large piece of tin foil and continue to cook.
Remove to a tray and tent with foil. Let rest 10 minutes. Slice and serve.