Flap steak is cut from the bottom sirloin and is sometimes call beef loin tip. It is less tender than more expensive steaks, but has a good beef flavor. It is ideal for marinating and needs to be cooked quickly on high heat to medium rare. Cut the cooked steak thinly across the grain before serving. This is the cut of beef that is often used for carne asada. In Hispanic grocery stores, it is often called steak ranchera.
Combine the citrus juices in a measuring cup. Add an equal amount of olive oil. Put the juice, oil, spices, garlic and soy sauce in a resealable plastic bag and mix well. Add the flap steak and seal the bag, squeezing out the air. Marinate up to 24 hours in the refrigerator, occasionally turning the bag.
Preheat the grill to high heat. Remove the steak from the plastic bag and discard the marinade.
Grill the flap steak over high heat about three minutes per side, until the steak is browned on both sides. Use the instant-read thermometer to test for doneness. Insert it from the side into the center of the steak. Cook to 140 degrees Fahrenheit for medium rare. Remove the steak from the grill and allow it to rest for 10 minutes before serving.
Slice the steak thinly across the grain. Serve it with salsa, tortillas, hot refried beans, salad and tomatoes.