"For the burgers• 1½ pounds fresh tuna steaks, cut into 1/2-inch dice• 2 tablespoons Dijon mustard• 1 tablespoon chipotle pepper in adobo purée• 1 tablespoon honey• 3 tablespoons canola oil• 2 green onions, green and pale-green part, thinly sliced• Salt and freshly ground pepper• 4 kaiser rolls• 2 ounces watercress• 1 red onion, peeled, halved and thinly sliced."