In a small saucepan, bring the soy sauce, sake, mirin and sugar to a boil. Cook over medium-high heat until the sauce is thickened and reduced by half (¼ cup), about 5 minutes.
Remove the sauce from the heat. Transfer half of the sauce to a bowl; cover and keep warm.
PREPARE THE TUNA and VEGETABLES:
Spray a broiler pan or grill rack with nonstick spray and preheat the broiler or grill.
Heat a large nonstick skillet over medium-high heat. Swirl in the oil, then add the ginger and garlic. Cook, stirring, until fragrant, less than 1 minute. Add bell peppers, snow peas, and carrots; cook, stirring, until vegetables are tender-crisp, about 4 to 5 minutes.
Meanwhile, broil or grill the tuna 5 inches from heat source, turning occasionally and brushing with half of the sauce, until golden brown and just slightly pink in the center, about 3 minutes on each side.
Divide the vegetable mixture among 4 plates. Top each serving with a piece of tuna and drizzle with the remaining sauce; serve immediately.
Notes
If you are preparing this recipe as GLUTEN FREE, just make sure you are using a brand of soy sauce that is designated as GF.