Drain salmon well, flake into a bowl, and remove large bones if desired.Add egg, onion, cornmeal, mayonnaise, mustard, salt, pepper, and garlic powder. Mix gently.Shape mixture into 6 even patties.Heat oil in a skillet over medium heat.Fry patties for 3 to 4 minutes per side until golden and crisp.Drain briefly on paper towels and serve warm.