Trim excess fat and silverskin from the skirt steak, then divide it into 3-4 even pieces.
Season the steak pieces evenly with kosher salt and place them in a gallon-sized resealable bag.
In a bowl, whisk together olive oil, red wine vinegar, soy sauce, Worcestershire sauce, Dijon mustard, garlic, thyme, and rosemary until emulsified.
Pour the marinade over the steaks in the bag, squeeze out air, seal, and massage to coat evenly.
Refrigerate and marinate the steaks for at least 2 hours and up to 6 hours, flipping occasionally.
About 30 minutes before grilling, remove steaks from marinade and let them rest at room temperature on a plate.
Preheat grill to high heat or heat a grill pan or cast iron skillet to medium-high.
Pat steaks dry to prevent flare-ups, then season generously with ground pepper.
Place steaks on the grill or pan in a single layer with space between pieces; cook 3-4 minutes per side to achieve a deep char and an internal temperature of 125-130°F.
Transfer steaks to a cutting board and rest for 10 minutes to let juices settle.
Slice the steak against the grain into half-inch pieces and serve immediately.