Coat both sides of the tuna steaks with the coarse pepper and season with salt. In a large heavy skillet, heat 2 tablespoons of the olive oil over high heat until almost smoking. Add the tuna and cook until well seared, about 2 minutes per side; the tuna should be rare inside. Let the fish rest for a few minutes.
Meanwhile, in a large bowl, combine the fennel, onion, chives and tomato halves. Add the remaining 1 tablespoon olive oil, season with salt and fine pepper and toss well. Mound the salad on 4 large plates.
Using a very sharp knife, sliced the tuna across the grain 1/4-inch thick and arrange it on the salads. Drizzle the tomato vinaigrette all around and garnish with the fennel fronds and chive blossoms.