Martha shows actress Jamie Lynn-Sigler how to make beef paillards.
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.