In a small jar, combine olive oil, red wine vinegar, minced garlic, Dijon mustard, kosher salt, and black pepper. Seal and shake until smooth.
Cut the skirt steak in half, pat dry, and rub with marinade ingredients. Let it marinate for about 30 minutes.
Preheat grill to medium-high heat. Grill steak for 3 minutes on each side for medium-rare. Rest for 5-10 minutes before slicing thinly against the grain.
In a large bowl, toss arugula, sliced red onion, and half the vinaigrette. Distribute onto plates.
Layer tomato slices, olives, crumbled goat cheese, and grilled steak slices on top. Drizzle with remaining vinaigrette before serving.
Notes
For best results, allow the steak to marinate for at least 30 minutes and always let it rest after grilling to ensure juicy slices.