To prepare rice, cook rice according to package directions, omitting salt and fat. Stir in 1/4 cup onions, peanut sauce, ginger, and 1/4 teaspoon salt; keep warm.
To prepare tuna, heat oil in a large nonstick skillet over medium-high heat. Combine tuna and 2 tablespoons soy sauce in a bowl, tossing gently to coat. Sprinkle tuna with 1/4 teaspoon salt. Dredge edges of tuna in sesame seeds. Add tuna to pan, and cook 3 minutes on each side or until desired degree of doneness. Serve tuna over rice. Drizzle with 1 tablespoon soy sauce; sprinkle with 2 tablespoons onions.