Peel and finely chop 2 cups yellow onions into 1/4-inch pieces. Place in a colander and let drain for 5 minutes to remove excess moisture.
In a large mixing bowl, whisk together 3/4 cup flour, 1 tablespoon sugar, 2 teaspoons baking powder, 1 tablespoon cornmeal, 1/2 teaspoon salt, and 1/4 teaspoon pepper until well combined with no lumps.
Pour in 3/4 cup whole milk and stir until you have a smooth, thick batter. The consistency should coat the onions but still be pourable.
Fold the drained onions into the batter until evenly coated. Let the mixture rest for 5 minutes while you heat the oil.
Heat 1/2 inch of vegetable oil in a large skillet over medium-high heat until it reaches 350-375°F. Test by dropping a tiny bit of batter—it should sizzle and turn golden in 30 seconds.
Using a tablespoon, drop spoonfuls of batter into the hot oil. Fry 4-5 fritters at a time without overcrowding the pan.
Fry for 2-3 minutes per side until deep golden brown and crispy. Flip gently with a slotted spoon.
Remove fritters and place on a wire rack over paper towels to drain. Sprinkle with salt while hot and serve immediately.
