Trim any large pieces of fat from the flank steak and pat the meat dry with paper towels. If the steak is uneven in thickness, pound gently to an even 1/2 inch using a meat mallet or the back of a heavy pan. Score any very dense areas lightly to help the marinade penetrate.

Make the spicy jalapeño marinade

In a medium bowl combine the finely chopped jalapeños (remove seeds for milder heat or keep some seeds for extra heat), minced garlic, soy sauce, lime juice, olive oil, brown sugar, ground cumin, smoked paprika, kosher salt, and black pepper. Whisk thoroughly until sugar dissolves and ingredients are well combined.

Marinate the steak

Place the prepared steak in a resealable plastic bag or a shallow dish and pour the marinade over it, making sure the meat is evenly coated. Massage the marinade into the steak briefly. Seal the bag or cover the dish and refrigerate for at least 30 minutes and up to 4 hours. (Do not marinate longer than 6 hours because the lime juice will begin to break down the meat excessively.) Remove the steak from the refrigerator 20 minutes before cooking to come closer to room temperature.

Finish & Garnish

Preheat and heat the pan or grill

Preheat a heavy skillet (cast iron preferred) over high heat until very hot, or preheat a grill to high. Add the vegetable oil to the skillet and swirl to coat once hot; if grilling, brush the grates with oil.

Sear the steak

Remove the steak from the marinade and blot excess marinade with paper towels (this ensures a good sear). Place the steak in the hot skillet or on the grill. For medium-rare, sear 3-4 minutes per side depending on thickness; for medium cook 4-5 minutes per side. Avoid moving the meat while searing to develop a deep crust. Use an instant-read thermometer to check: 125-130°F for medium-rare, 135°F for medium.

Rest and finish

Transfer the seared steak to a cutting board and tent loosely with foil. Add the unsalted butter to the still-hot skillet and swirl in any juices left from the steak; let the butter melt and become lightly browned (30–45 seconds), then spoon the butter over the tented steak. Rest the steak 8–10 minutes to allow juices to redistribute.

Slice and serve

Slice the steak thinly against the grain to maximize tenderness. Arrange on a serving platter or individual plates. Spoon any browned butter from the pan over the sliced steak. Garnish with chopped cilantro, thinly sliced red onion, and additional sliced jalapeño if desired. Serve with lime wedges on the side for squeezing over the meat.