From a now passed internet friend... these are the gold standard. I up the garlic, the jalapenos, and I use a mix of chipotle and anaheim chiles, which I toast myself, and then grind. Use skirt steak.
1/2 Onion -- halved, slice thin
2 Teaspoons Ground Cumin
2 Teaspoons Powdered Red Chiles
3 Jalapenos -- (pickled) chopped
2 Cloves Garlic -- chopped
1/4 Cup Lime Juice
2 Tablespoons Jalapeno Pickling Liquid -- * see note
1 Tablespoon Corn Oil
*This is the liquid used to pickle and flavor the jalapenos. It is basically white vinegar with added spices, and there is always ample liquid in the jar or can to use in this recipe, without leaving the remaining jalapenos dry.
Place half of the onions in the bottom of a nonreactive dish.
Mix the cumin, powdered red chiles, chopped jalapenos and garlic together in a small bowl, then rub on all sides of the meat.
Put the skirt steak into the dish, on top of the onions. Pour the lime juice and the jalapeno liquid over all areas to coat. Sprinkle the remaining onions on top of the meat. Cover and refrigerate at least 1 hour, but preferably overnight, turning once.
Preheat the grill until hot. Fajitas need to cook close to a very high heat source, in order to sear the outside but still leave the interior medium rare.
Brush or spoon the oil onto the meat surfaces. Grill or broil about 2 to 3 minutes on each side, or until the outside is brown and slightly charred, and the inside is still slightly pink. Remove the meat to a cutting board. Let sit 5 minutes before slicing to rest.