Score flank steak diagonally on both sides.
Place meat in a plastic bag; set in shallow pan.
Stir together cooking oil, lemon juice, garlic, salt, and pepper; pour over meat.
Let stand at room temperature for 2 to 3 hours or overnight in refrigerator, turning several times.
Remove meat from marinade; place on an unheated rack in a broiler pan.
Broil steak 4 inches from heat, 4 to 5 minutes per side for medium rare, brushing with marinade occasionally.
Meanwhile, in a skillet melt butter or margarine.
Add sliced onions; cook and stir till onion is tender.
Stir in snipped parsley.
Carve steak diagonally across the grain into very thin slices.
Arrange the steak slices on each slice of toasted French bread; spoon the onion mixture over steak sandwiches.