1) Rub both sides of flank steak with olive oil, all purpose rub, and red pepper flakes. Let it rest at room temperature for 30 minutes. Alternatively, you can rub it, wrap it in plastic and let it sit overnight in the fridge.
2) For black bean salad, just mix all the ingredients in a large bowl and season with salt and pepper. Set aside in the fridge until needed.
3) Heat grill on high heat and let heat for 10 minutes to get very hot.
4) Grill flank steak for 5-6 minutes per side for a medium rare steak.
5) Remove flank steak from grill and let rest for 5 minutes before slicing.
6) Slice flank steak across the grain and serve over black bean salad with flour tortillas.