On a work surface, dry the salmon thoroughly with paper towels and then remove any small bones still in the flesh. Transfer the salmon to a 9-by-13-inch baking dish. In a small bowl, stir the salt with the sugar, pepper, paprika, and sumac. Rub the spice mix all over the flesh and then wrap the whole dish in plastic wrap. Refrigerate the salmon for at least 24 hours, or up to 48 hours, to cure.
Meanwhile, for the marinade, use a Microplane grater to finely grate half the orange's zest into a small bowl. Using a vegetable peeler, peel the zest from the other orange half and slice it lengthwise into very thin strips. Juice the orange and measure out 6 tablespoons; reserve remaining juice for another use. Stir the orange juice into the grated zest along with the olive oil and honey until the honey dissolves.
Unwrap the salmon and transfer to a cutting board. Gently pat dry the flesh of the salmon and then cut across the grain into 1⁄4-inch-thin slices. Arrange the slices to overlap on a large serving platter, and then spoon the marinade over the slices. Sprinkle with the orange zest strips and let stand for 30 minutes in a cool place to meld flavors before serving with toast.