Mix the dry ingredients: In a large bowl, whisk together cornmeal, flour, baking powder, baking soda, salt, and sugar until well combined.
Combine the wet ingredients: In a separate bowl, beat the egg, then stir in buttermilk, diced onion, and melted butter until smooth.
Form the batter: Gently whisk the dry ingredients until they are barely incorporated after adding the wet components. Although thick, the batter should be scoopable. Avoid overmixing to keep the hush puppies tender.
Heat the oil: In a deep skillet or Dutch oven, pour enough vegetable oil to reach a depth of about 2 inches. Heat to 350°F (use a thermometer for accuracy).
Fry the hush puppies: Using a small cookie scoop or spoon, drop tablespoon-sized portions of batter into the hot oil, being careful not to overcrowd the pan. Fry until golden brown and crispy, 2 to 3 minutes per side.
Drain and serve: Remove hush puppies with a slotted spoon and drain on a paper towel-lined plate. Serve hot with tartar sauce, remoulade, or a sprinkle of sea salt.