In a big bowl, combine the lime juice, garlic, cumin, salt, 1/3 cup of vegetable oil and tequila. Mix the ingredients until they form a uniform, smooth texture.
Immerse the beef fajita into the bowl then leave for about 2-6 hours. 4 hours is ideal. Following that, remove your meat. Avoid rinsing it.
Preheat your cast iron skillet or grill.
Cook steak fajitas for three to five minutes per side. Do not overcook. If you're concerned about the marinade, don't worry; everything will be cooked.
Allow the meat to rest for at least 5 minutes before slicing the grain into the strips. Cooking time can sometimes be as long as 9 minutes per side. You need a medium-rare steak. It is optional to cook the meat for 2 minutes after slicing it into strips.
Sauté the onions and bell peppers in 2 tablespoons vegetable oil over medium-high heat. Serve the Lupe Tortilla Fajitas alongside warm tortillas (flour or corn) and vegetables. Chipotle sauce, Guacamole, salsa, pico de gallo, cheddar cheese, and sour cream can be served as sides. You could also serve tortilla chips or nachos.