Whisk together cornmeal, flour, sugar, baking soda, baking powder, Cajun seasoning, salt, and black pepper in a large bowl.
In a separate medium bowl, add your milk and vinegar; let sit for 5 minutes. Whisk in vegetable oil, egg, and onion. Add wet ingredients to dry ingredients; mix just until incorporated.
Heat 1 1/2 inches of oil in a Dutch Oven or heavy skillet to 365 degrees. Using two small spoons, scoop a rounded spoonful onto one spoon and gently push it off with the second spoon. Fry for 3-5 minutes or until golden brown. Drain on paper towels. Serve promptly.