Combine the onion, flour, cornmeal, baking powder, salt, sugar and onion powder in a bowl. Whisk the egg and milk in another bowl. Whisk the egg mixture into the flour mixture until smooth. Cover and chill 30 minutes.
Heat 2 inches vegetable oil in a large pot over medium heat until a deep-fry thermometer registers 350 degrees F. Working in batches, carefully drop heaping tablespoonfuls of batter into the hot oil. Fry, stirring occasionally, until golden, about 8 minutes per batch (return the oil to 350 degrees F between batches). Transfer to a paper towel-lined baking sheet using a slotted spoon. Sprinkle with salt. Let rest a few minutes before serving.