Combine the flour, cornmeal, sugar, and baking powder in a mixing bowl. Stir in the milk, then fold in the chopped onions until the batter comes together and the onions are evenly coated.

Pour 1/2 to 1 inch of oil into a deep-sided skillet and warm it over medium-high heat until it reaches about 175°F (the oil should shimmer and move freely).

Using a tablespoon, drop spoonfuls of batter into the hot oil, spacing them apart and gently flattening each mound a little so they cook evenly.

Fry until the underside is golden brown, then flip and cook the other side until it is also golden and the fritters are cooked through. Remove and let drain on a paper towel–lined rack.

Serve the fritters warm with sour cream or your preferred dipping sauce, if desired.