Whisk together flour, cornmeal, salt, and pepper in a medium bowl. Stir in onion and jalapeno, if using. Slowly pour in milk while stirring and add just enough to make a wet batter.
Using two teaspoons, scoop batter out of bowl with one teaspoon and carefully push the batter off of the teaspoon with the second teaspoon into the hot oil. Hush puppies will rise back to the top of the hot oil. Allow to turn golden brown on one side and then gently flip the hush puppies onto the other side for browning with a slotted spoon. Remove from hot oil and drain on a paper towel lined plate for serving.
If you make fried catfish, then use the leftover breading used to batter the fish and add in onion, jalapeno, and milk.