Prepare the flap steak by cutting it against the grain to ensure tenderness. Set aside.
In a bowl, combine orange juice, lime juice, Worcestershire sauce, dark beer, jalapenos, garlic, spring onions, and cilantro to create the marinade. Submerge the steak in the marinade and refrigerate for 2-3 hours.
Preheat your grill or cast iron grill pan. Remove the steak from the marinade, pat dry, and season with salt and pepper.
Grill the steak over high heat for 3-4 minutes on each side, or until you achieve a nice char and the desired doneness. Let the steak rest for 8-10 minutes before chopping it into small pieces.
Warm the tortillas on the grill. Assemble the tacos by placing the chopped steak on the tortillas, followed by chile de arbol salsa, avocado slices, chopped onions, and cilantro.
Serve the tacos with lime wedges and grilled spring onions on the side. Enjoy with family and friends for an authentic Mexican experience.