In a large bowl, whisk together the dry ingredients: cornmeal, flour, sugar, baking soda, baking powder, Cajun seasoning, salt, and pepper. In a separate medium bowl, combine the room temperature milk and white vinegar. Let it sit for 5 minutes; it will curdle slightly, creating a buttermilk substitute. To this mixture, whisk in the egg, vegetable oil, and grated onion until well combined.
Pour the wet mixture into the bowl of dry ingredients. Stir gently with a spoon or spatula only until the ingredients are just combined. It’s important not to overmix; a few lumps in the batter are perfectly fine. Overmixing can develop the gluten in the flour and result in dense, tough hush puppies.
Cover the bowl and let the batter rest for 10-15 minutes while you heat the oil. This step allows the cornmeal to fully absorb the liquid, which thickens the batter and improves the final texture.
Pour about 2-3 inches of oil into a heavy-bottomed pot, Dutch oven, or deep skillet. Heat the oil over medium-high heat until it reaches 365-370°F (185-188°C). Maintaining the correct oil temperature is the key to success. If it’s too hot, the outside will brown too quickly, leaving the inside raw. If it’s too cool, the hush puppies will absorb too much oil and become greasy.
Using a small cookie scoop or two spoons, carefully drop rounded tablespoon-sized portions of batter into the hot oil. Do not overcrowd the pan; fry in small batches of 5-6 at a time. They should sizzle immediately. Fry for 3-5 minutes, turning them occasionally with a slotted spoon, until they are a deep golden brown on all sides and cooked through.
Use a slotted spoon to remove the cooked hush puppies from the oil, allowing excess oil to drip off. Transfer them to a plate lined with paper towels to drain further. If you are making multiple batches, you can keep the finished hush puppies warm on a baking sheet in a 200°F (95°C) oven. Serve immediately while hot.