Prepare plate for draining hush puppies. Line a large plate with a double layer of paper towels.
Combine the dry ingredients. Whisk 1 1/2 cups cornmeal, 1/2 cup all-purpose flour, 1 1/2 teaspoons kosher salt, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon granulated sugar, if using, together in a large bowl.
Prepare the wet ingredients and add to the dry. Grate 1/2 small yellow onion on the large holes of a box grater (about 1/4 cup) and add to the bowl. Add 1 large egg and 1 cup buttermilk.
Mix and rest the batter. Stir until combined. Set aside for 15 minutes while the oil heats.
Heat the oil and prepare a plate for draining the hush puppies. Add enough peanut or vegetable oil to a Dutch oven for a depth of 1 1/2 inches (about 4 cups). Heat over medium-high heat to 365ºF.
Fry the hush puppies in batches. Use a small ice cream scoop to drop golf ball-sized rounds (about 2 tablespoons each) of the batter into the oil, about 5 at a time. Make sure not to crowd the pot which will cause the oil temperature to drop. Turn the hush puppies frequently with a slotted spoon or tongs until deeply browned on all sides, 3 to 4 minutes.
Drain the hush puppies and serve. Transfer with a slotted spoon to the paper towels to drain. Repeat with the remaining batter, adjusting the heat to maintain the oil temperature. Serve hot with salted butter.