Stir together sugar,mustard,salt,cumin and pepper in small bowl. Unwrap fish and leave it on paper you bought it in, placing it so side that had skin is down. Press Rub mixture evenly onto fish.
Heat oil in large nonstick or cast-iron skillet over medium heat. When hot, place fish rub-side down in pan. Cook until rub dissolves and darkens slightly (being careful not to burn), about 4 minutes. Flip fish and cook to medium doneness, about 1 minute.