To begin, slice your flank steak into thin slices. Sprinkle with the all-purpose seasoning.
Slice your onion and bell pepper into strips. In a large skillet, melt a little butter.
Sauté the onion and pepper for just a couple minutes until just tender. Set those aside and add in the meat. Cook on high heat until just done. Place some of the onions, pepper, and steak on a tortilla.
Top with some cheese and guacamole. Fold it over and place in a panini press for about a minute and a half to melt it all together.