For vinaigrette, in a screw-top jar combine vinegar, oil, garlic, salt, and pepper; cover and shake well. Trim fat from steak. Remove 1 tablespoon vinaigrette from jar and brush evenly onto both sides of the steak. Press thyme and rosemary onto both sides of the steak. Use a clean brush to coat both sides of onion slices with some of the remaining vinaigrette, reserving the rest; set aside.

Preheat gas grill. Reduce heat to medium. Place steak on the grill rack directly over heat. Cover and grill until steak is desired doneness, turning once halfway through grilling. [Allow 20 to 24 minutes for medium rare (145 degrees F) or 24 to 28 minutes for medium (160 degrees F).] While the steak is grilling, add onion slices to grill. Cover and grill for 6 to 8 minutes or until tender, turning once halfway through grilling.

Divide the mesclun among 4 dinner plates. To serve, thinly slice the steak across the grain. Separate onion slices into rings. Arrange warm steak slices and onion rings on top of mesclun. Drizzle with the reserved vinaigrette. Top with tomatoes and cheese. Makes 4 servings.

grill steak directly over medium coals for 20 to 24 minutes for medium-rare or 24 to 28 minutes for medium, turning once halfway through grilling. Add onion slices to grill the last 6 to 8 minutes, turning once after 4 minutes. Prepare dressing and salad as above.