Stir together the sugar, salt, pepper, cumin, and mustard in a small bowl. Press the sugar mixture evenly onto the top surface of the fish.
Heat the oil in a large nonstick skillet over medium heat. Cook the fish, rub side down, for 4 minutes or until the rub dissolves and darkens slightly (being careful not to burn). Flip the fish and cook for 3 minutes or until the fish is opaque.
This is awesome on the grill. DO NOT OVERCOOK THE FISH!