Line a large plate with a double layer of paper towels and set it near the stove for draining the fried hush puppies.
In a large bowl, whisk together 1 1/2 cups fine cornmeal, 1/2 cup all-purpose flour, 1 1/4 teaspoons kosher salt, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon Cajun or Creole seasoning, 1/2 teaspoon garlic powder, and 3/4 teaspoon granulated sugar.
Add the grated 1/2 small yellow onion (about 1/4 cup) to the dry ingredients. Add 1 large egg and 1 cup buttermilk, then stir gently until just combined — do not overmix.
Let the batter rest for 15 minutes at room temperature.
While the batter rests, pour 4 cups canola oil or vegetable oil into a deep pot or Dutch oven to a depth of about 1 1/2 inches. Heat the oil over medium-high heat until it reaches 365°F.
Using a small ice cream scoop or two spoons (about 2 tablespoons per hush puppy), carefully drop rounded dollops of batter into the hot oil. Fry in batches without crowding the pot.
Turn the hush puppies gently with a slotted spoon as they cook so they brown evenly, frying each batch about 3 to 4 minutes total, until golden brown on all sides.
Transfer cooked hush puppies to the prepared paper towel–lined plate to drain. Let the oil return to 365°F before frying the next batch.
Serve the hush puppies hot.