In two separate bowls, toss the scallops and tuna with the lime juice and let sit for 10 minutes. Meanwhile, peel and section the grapefruit, cutting around the membrane. (See Cook's Tip.) Chop into ¼- to ½-inch (.6- to 1.2-cm) pieces. Peel and slice the mango into ¼-inch (.6-cm) slices. Drain the scallops and the tuna, reserving the lime juice. In a bowl, gently toss together and then toss with the soy sauce, coriander, sesame oil, and jalapeño pepper. When the fish is thoroughly coated, fold in the grapefruit and mango. Taste, and add some of the reserved lime juice, if necessary.
Using your hands, mound on six plates and serve immediately.
COOK'S TIP
To get citrus slices without the membrane, cut a small piece off the top and bottom of the grapefruit. Set the fruit on a cutting board and, using a sharp knife, follow the contour of the grapefruit, cutting away the skin and the thin membrane. Cut into the grapefruit close to the membrane on both sides of each section of fruit.
Photo by Chris Freeland.