Place tuna in a glass dish. Prepare the vinaigrette by whisking together the wine, olive oil, red wine vinegar, rosemary, pepper, salt and garlic. Pour 2 tablespoons over fish and turn to coat. Marinate 15 to 30 minutes, turning once. Reserve remaining vinaigrette for salad dressing.
Coat barbecue with cooking spray. Place tuna on grate 10-15cm over hot coals. Cover with lid or tent with foil. Cook, turning once just until tuna begins to flake easily when tested with a fork, about 7 minutes. Discard marinade.
Arrange salad greens on 4 plates. Place hot tuna on greens and add cherry tomatoes. Stir remaining vinaigrette and drizzle over salads.