POUND each steak with meat mallet to 1/8-inch thickness; set aside. Combine 2 Tbsp. of the dressing, the lemon juice and garlic. Pour over steaks in glass dish; cover. Refrigerate at least 1 hour to marinate. Remove steaks from marinade; discard marinade.
COOK 2 of the steaks in 1 Tbsp. of the remaining dressing in large skillet on medium-high heat 2-1/2 min. on each side or until cooked through. Remove steaks from skillet; place on bottom halves of 2 of the rolls. Cover to keep warm. Repeat with remaining 1 Tbsp. dressing and remaining 2 steaks.
ADD onions to skillet; cook 5 min. or until lightly browned and tender, stirring occasionally. Add parsley; mix well. Spoon evenly over steaks; cover with top halves of rolls. Brush both sides of each roll with butter.
HEAT two large skillets on medium-high heat. Place sandwiches in one of the skillets. (If all four sandwiches do not fit in the skillet, cook sandwiches in batches.) Place second preheated skillet on top of sandwiches; gently press down skillet to to flatten sandwiches slightly. Cook 4 to 5 min. on each side or until both sides of rolls are crisp and golden brown. Serve warm.