Paillards are boneless pieces of meat pounded thin for quick and easy cooking. Here, marinated beef paillards are served with an arugula salad and pickled onions.
Cut open two sides of a resealable plastic bag. Place 1 steak in bag; pound to 1/4-inch thickness with a meat pounder. Transfer to a large baking dish. Repeat with remaining steaks. Add marinade. Chill 2 hours.
Place onion in a small bowl with red-wine vinegar, and season with salt; toss to combine.
Heat a grill or grill pan. Place arugula, celery, lemon juice, and olive oil in a medium bowl; toss to combine. Season with salt and pepper. Remove meat from marinade; discard marinade. Season meat with salt and pepper. Grill until browned, about 1 minute on each side. Place two pieces of beef on each plate; sprinkle with capers. Top with arugula salad and pickled onions.