Preheat oven to 475°F. Grate 1 teaspoon grapefruit peel; reserve. Cut off peel and pith from grapefruit; cut fruit into six 1/4-inch-thick slices and discard seeds.
Mix shallots, tarragon, and grated grapefruit peel in small bowl to blend. Drizzle olive oil over bottom of medium roasting pan; heat pan in oven 3 minutes. Sprinkle shallot mixture over oil. Place tuna steaks in single layer atop shallot mixture; sprinkle with salt and pepper. Roast 5 minutes. Turn tuna steaks over. Top each with 1 slice grapefruit. Roast tuna to desired doneness, about 5 minutes longer for medium.
