Chris's dish is great as long as you can get spanking fresh, sushi-quality tuna, but home cooks may have a hard time finding it-or paying for it. My alternative recipe is less "pure" but works quite well with ordinary supermarket tuna, which is likely not fabulous enough to eat raw or very rare, but still good. You can use the grill or stovetop for this method, both with great success.
1/3 cup sake or slightly sweet white wine
1/3 cup mirin (or use 2 tablespoons honey mixed with 2 tablespoons water)
1/3 cup soy sauce
2 tablespoons sugar
4 (6-ounce) tuna steaks
1a. If you're grilling, start a charcoal or gas grill; the fire should be moderately hot, and the rack should be about 4 inches from the heat source. Combine the sake, mirin, soy, sugar, and 2 tablespoons water in a small saucepan and heat gently, stirring to dissolve the sugar. Grill the fish on one side until brown, then begin brushing it frequently with the sauce and turn it once or twice more, until nicely browned on both sides and cooked to the stage of doneness you prefer. Brush once more with the sauce, then serve.
1b. If you are cooking on a stovetop, preheat a large non-stick skillet over medium-high heat for about 2 minutes, then add the tuna. Brown quickly on both sides, not more than 2 minutes per side. Transfer the fish to a plate and turn the heat to medium. Add 2 tablespoons water, followed by the sake, mirin, sugar, and finally soy sauce. Stir to blend and, when the mixture is producing lively bubbles and beginning to thicken, about 10 minutes, return the tuna to the pan.
2b. Cook about 2 minutes more, turning the tuna a couple of times to glaze it with the sauce. By that time the fish will be adequately cooked (it should still be slightly rare in its center, but you can cook it longer if you prefer). Serve hot or at room temperature.