In a medium mixing bowl, whisk together the olive oil, fresh lemon juice, minced garlic, chopped fresh parsley, chopped fresh oregano, fresh thyme leaves, Dijon mustard, salt, black pepper, and red pepper flakes (if using) until well combined.

Place the skirt steak into a large Ziploc bag or a shallow non-reactive dish. Pour the prepared marinade over the steak, ensuring it's fully coated. Seal the bag (or cover the dish) and refrigerate for at least 2 hours, or preferably 4-12 hours for maximum flavor and tenderness.

About 30-45 minutes before you plan to cook, remove the marinated skirt steak from the refrigerator. Allow it to come closer to room temperature, which promotes more even cooking.

Preheat your grill to high heat (450-500°F / 230-260°C) or a heavy-bottomed cast iron skillet over medium-high heat until smoking lightly. Lightly oil the grill grates or the skillet to prevent sticking.

Remove the steak from the marinade, letting any excess marinade drip off (discard any remaining marinade). Place the skirt steak onto the preheated hot grill or skillet.

Cook for 3-5 minutes per side for a medium-rare doneness (internal temperature 130-135°F / 54-57°C), or 5-6 minutes per side for medium (internal temperature 135-140°F / 57-60°C). Skirt steak is thin and cooks very quickly, so watch it closely.

Transfer the cooked skirt steak to a clean cutting board and tent loosely with aluminum foil. Let it rest for 5-10 minutes. This crucial step allows the juices to redistribute throughout the meat, ensuring a more tender and flavorful result.

Slice the skirt steak thinly against the grain. Skirt steak has a very distinct grain; cutting perpendicular to these muscle fibers is essential for maximum tenderness. Serve immediately with your favorite sides.