12 wooden or metal 8-inch skewers (see Cook's Note)
For the beef: In a resealable plastic bag or glass bowl, combine the olive oil, garlic, herbes de Provence, salt, pepper, and beef. Refrigerate for at least 6 hours or overnight.
Place a grill pan over medium-high heat or preheat a gas or charcoal grill. Remove the meat from the marinade and season with salt. Thread 4 pieces of meat onto each skewer and grill for 3 to 4 minutes on each side for medium-well. Arrange the skewers on a platter.
For the relish: In a small bowl, combine the onion, sun-dried tomatoes, corn, and olive oil. Refrigerate until ready to serve. Just before serving, add the avocado, feta cheese, and parsley. Gently toss and season with salt and pepper, to taste. Serve the relish alongside the beef skewers.
To prevent wooden skewers from burning, soak in water for 30 minutes prior to grilling.