Though Tex-Mex-style fajitas are unknown in Mexico, grilled skirt steak is eaten with tortillas in Nuevo León, under the name arrachera al carbon.
3 small yellow onions, peeled and halved lengthwise
3⁄4 cup plus 1 tbsp. vegetable oil
1 tbsp. worcestershire sauce
1⁄4 tsp. liquid smoke
1 bay leaf
1 1⁄2 tsp. freshly ground black pepper
1 2-lb. skirt steak, cut into 3" pieces
1 green bell pepper, stemmed, cored, and thickly sliced
1 red bell pepper, stemmed, cored, and thickly sliced
12 scallions, trimmed
1 tomato, cored and quartered
6" homemade or store-bought Flour Tortillas
1. Finely chop 1 of the onion halves and put into a large deep glass or ceramic dish. Add 3⁄4 cup of the oil, worcestershire sauce, liquid smoke, bay leaf, black pepper, and salt to taste and mix well. Add meat to dish and turn in marinade until well coated. Cover dish with plastic wrap and marinate meat in the refrigerator for at least 8 hours and up to 12 hours.
2. After steak has marinated, heat a charcoal grill until coals are hot. Remove meat from dish, discarding marinade. Grill meat over hot coals, turning once, 4–6 minutes for medium rare. (You may also cook meat in a grill pan on the stove over high heat.) Transfer meat to a cutting board and set aside.
3. Thickly slice the remaining 2 onions lengthwise and set aside. Heat the remaining 1 tbsp. oil in a large skillet over medium-high heat until hot but not smoking. Add onions and bell peppers, season to taste with salt, and cook, stirring often, until vegetables are lightly charred but still crunchy, 3–4 minutes.
4. Meanwhile, thinly slice meat against the grain, add to skillet with vegetables, and stir until heated through, 1–2 minutes. Divide fajitas equally among four heated cast-iron fajitas platters or large heated plates; garnish with scallions and tomato wedges. Serve with warm tortillas, if you like.