Whisk together the flour, cornmeal, baking powder, sugar, baking soda, salt
and pepper in a large bowl.
Stir together buttermilk, eggs, and grated onion with their juice in a
medium bowl. Pour over dry ingredients and stir together until just
combined. Set aside to rest for 1 hour.
Pour oil to a depth of 2 inches in a 6-quart Dutch oven and heat over
medium-high heat until a deep-fry thermometer reads 350 degrees F.
Using a tablespoon, drop small rounds of batter into the oil, making
sure to not crowd the pan. Cook, stirring occasionally and flipping halfway
through, until golden on the outside and crisp, about 3–4 minutes.
Remove hush puppies from the oil and drain on paper towels.