Prepare the marinade. In a ziplock bag or storage container, add the soy sauce, pineapple juice, lime juice, cayenne pepper and flank steak. Marinate overnight or at least a minimum of 4 hours.
Remove the steak from the marinade. If the cut is thick, butterfly it and slice in to thin strips - about 1.5-2 inches long.
Heat grill pan or cast iron skillet over high heat. Once hot, sear the steak pieces 5 to 7 minutes, stiring occasionally. Remove from heat.
In a small skillet, warm the tortilla over medium heat, top with cheese and cover with a lid. Heat for 1 to 2 minutes until cheese is melted and tortilla is just slightly crispy. Top with you favorite taco toppings!
Marinate for a minimum of 4 hours and a maximum of 36 hours.
For a more intense marinade flavor, cut the flank steak before marinating.
If using a regular skillet (not a preseasoned grill pan or cast iron) grease the pan with a small bit of canola oil before add the steak.