Prepare a charcoal grill. When the charcoal is hot and has a coat of white ash over it push all of the coals to one side of the grill. Pat the salmon pieces dry. Salt very lightly on both sides. Place the salmon pieces skin side down, on the cool side of the grill. Cover the salmon, in groups of two or three if necessary, with a small domed cover, such as a stainless steel mixing bowl. It is important to use a domed cover to enclose the fish. It is the slow hot air circulation that cooks the fish, not the hot coals.
Grill the salmon for 15 minutes without turning. Meanwhile, mix the mustard powder and sugar and whisk in the water until it has reached the consistency of heavy cream. During the last 3 or 4 minutes of cooking lift the domes and brush each piece of fish with olive oil. Replace the domes to finish the cooking. The salmon will be a whitish pink color and very tender to the touch and the skin will be brown and very crisp.
Serve each piece of salmon skin side up with a simple salad, if desired.