Heat 2–3 inches of vegetable, peanut, or canola oil in a deep, heavy pan to 350–365°F. Use a thermometer if possible, since accurate oil temperature helps the hush puppies cook through without becoming greasy or overly dark.

Whisk the cornmeal, flour, sugar, baking powder, baking soda, salt, and black pepper in a large bowl. Stir in the finely minced yellow onion so it is evenly distributed through the dry mixture.

In a separate bowl, whisk together the egg and buttermilk. Pour the wet mixture into the dry ingredients and stir only until combined; the batter should be thick and scoopable, similar to very thick muffin batter.

If the batter seems too stiff to scoop, add buttermilk 1 tablespoon at a time. Let the batter rest for 5–10 minutes so the cornmeal can hydrate and the mixture can thicken into a cohesive batter.

Drop rounded teaspoons or small tablespoons of batter into the hot oil, working in batches and leaving plenty of room between pieces. Keep each hush puppy close to 1 tablespoon so the outside browns at the same pace the center cooks.

Fry for 3–5 minutes per batch, turning occasionally with a slotted spoon. Continue until the hush puppies are puffed and evenly deep golden brown, then transfer them to a wire rack or paper towels to drain.

Serve the hush puppies hot, while the edges are crisp and the centers are tender. Repeat with the remaining batter, allowing the oil to return to 350–365°F between batches.