Trim the skirt steak of any membranes and surface fat. Slice the meat diagonally (across the grain) into thin strips.
Heat 1 tablespoon of oil in a 12″ cast iron skillet over medium-high heat. Add the prepared meat in batches and brown quickly, about 2–3 minutes per side. Do not crowd the pan as this will trap steam and interrupt the browning process which creates flavor (Maillard reaction). Remove the meat to a plate and set aside.
In the same skillet, add another scant tablespoon of oil, if needed. Add sliced bell pepper and onion and cook briefly until lightly browned at the edges. They should remain crisp at this point as they will be cooked further in the next step.
Return the browned meat to the skillet, along with any juices that have accumulated on the plate. Add the soy sauce, jalapeño juice, tomato juice, jalapeños, black pepper, and cumin. Stir to combine and simmer for 3–5 minutes until heated through and flavors are well blended.
Serve with warm tortillas and your choice of salsa, sour cream, cheese, lettuce, additional jalapeños, or other condiments.
Optional presentation: Preheat a cast iron serving skillet to 350°F and transfer the fajitas to it for immediate serving.
Notes
Brown the steak in batches — overcrowding the pan traps steam and prevents the browning (Maillard reaction) that builds flavor.
The vegetables should still be crisp after flash-frying since they cook further when simmered with the sauce.
Always slice skirt steak diagonally across the grain for the most tender strips.
Save any juices that accumulate on the plate while the meat rests and add them back to the skillet when you combine everything.
For a sizzling restaurant-style presentation, preheat a cast iron serving skillet to 350°F and transfer the fajitas to it before bringing to the table.