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 cooking grate with oil spray and place on barbecue to heat 1 minute. Place tuna on barbecue 10 to 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.
Alternative cooking methods
The tuna steaks can also be cooked under the grill.