How to Make It
Mix together the oil, vinegar, mustard, salt, and pepper in a large, shallow dish.
If desired, wrap a rosemary sprig around edge of steak and tie ends securely with kitchen string. Repeat with other steaks. Place steaks in marinade and turn to coat. Cover with plastic wrap and refrigerate 30 minutes or overnight.
Lightly coat clean grids of grill with cooking oil spray. Heat the grill to medium-high heat. Lightly brush onion and tomato halves with oil. Place onions on grill, cut side down, and grill 10 minutes with the lid down. Turn and cook 5 minutes more; transfer to a plate and keep warm.
Place steaks on grill and cook, without turning, 4 minutes with the lid down. Turn steaks and place tomatoes, cut side down, on grill. Cook steak and tomatoes, with lid down, 4 minutes. Transfer steak to plates along with the onions and tomatoes.