Grilled Tuna with Wasabi-Ginger Vinaigrette

3 tablespoons low-sodium soy sauce
1/4 cup lime juice
2 teaspoons sugar
2 teaspoons wasabi powder, mixed with 2 tablespoons water
2 teaspoons minced ginger
2 tablespoons canola oil
2 scallions, finely chopped
4 grilled tuna steaks, 2 inches thick, about 4 ounces each
1 medium red onion, halved, sliced very thinly
2 bunches watercress, tough stems removed

Combine the soy, lime juice, sugar, wasabi, ginger, canola oil and the scallions in a large bowl and whisk. Taste for seasoning. Set aside.

Cut the grilled tuna into large chunks and toss with the vinaigrette. Set aside. (This recipe can be made ahead and refrigerated overnight.)

Combine the red onion and watercress in a large bowl and toss well. Place the onion-watercress mixture in 4 large plastic containers and spoon the tuna on top. Cover and bring to work.