Build a charcoal fire or preheat a gas grill. In a small bowl, combine the couscous, cashews, chopped scallions and orange peel; set aside.
In a measuring cup, whisk together the orange juice, sherry, soy sauce, mirin, 1 tablespoon sesame oil and the ginger; set aside.
Tear off four 2-foot-long sheets of heavy-duty foil and fold each in half to form a rectangle; brush the center of each lightly with sesame oil. Divide the couscous mixture evenly among the 4 rectangles and top each pile with 1 cup baby spinach and a fish fillet; season with salt and pepper. Fold the edges of the foil up and inward to start forming a packet. Whisk the grilling sauce and pour about 1/4 cup into each packet. Roll up the edges of the foil together to seal.
Grill the packets over indirect medium heat, with the lid down, for 25 minutes. Let the fish rest for 3 minutes before opening the packets. Garnish with the remaining scallion matchsticks.