Rinse tuna; pat dry. Spread garlic on both sides of steaks; sprinkle with salt and pepper. Pour oil into an 8- to 10-inch nonstick frying pan over medium-high heat. When hot, add tuna. Cook, turning once, until lightly browned on both sides, about 1 minute per side. Pour sake and 1 tablespoon soy sauce around steaks; remove from heat. Let cool, turning fish often.
Meanwhile, in a small bowl, mix tomatoes, green onion, cilantro, lemon juice, and remaining tablespoon soy sauce.
Lift tuna from sake mixture, reserving juices. Cut fish across the grain into 1/4-inch-thick slices and lay on plates. Garnish with the salsa and avocado slices. If desired, spoon pan juices equally over tuna (otherwise discard).