Rinse fish and pat dry. Sprinkle lightly all over with salt and pepper and place on a 12- by 15-inch baking sheet. Bake in a 350° regular or convection oven until opaque but still moist-looking in center of thickest part (cut to test), about 10 minutes. With a wide spatula, transfer fish to a bowl. Chill until cold, at least 1 hour; if desired, cover when cold and chill up to 1 day.
Shred fish with fork. Add pineapple and 2 tablespoons juice, cashews, coconut, green onions, and mayonnaise; mix well.
Mound halibut salad equally on bottoms of 4 sandwich rolls. Top with salad mix and roll tops.
Nutritional analysis per sandwich.