Peel and slice onions into 1/2-inch thick rings, then separate them. In a shallow dish, whisk together 1 cup of flour, cornstarch, baking powder, salt, pepper, and paprika. In a separate bowl, whisk the egg and cold milk until combined. Add the remaining 1/2 cup of flour to the wet mixture and whisk until a smooth batter forms.

Pour the oil into a large Dutch oven or heavy-bottomed pot to a depth of at least 2 inches. Heat over medium-high heat until a deep-fry thermometer registers 375°F (190°C). Set a wire rack over a baking sheet nearby.

Working one ring at a time, dredge it in the dry flour mixture, shaking off the excess. Dip it into the wet batter, allowing the excess to drip back into the bowl. Finally, dredge it once more in the dry mixture for an extra-thick coating. Place on a separate baking sheet and repeat with remaining rings.

Carefully place about 4-5 coated rings into the hot oil. Do not overcrowd the pan. Fry for 2-3 minutes per side, until deep golden brown and crispy.

Using tongs or a spider strainer, remove the onion rings from the oil and transfer them to the prepared wire rack to drain. Immediately sprinkle with salt. Make sure the oil returns to 375°F before frying the next batch.

Serve the homemade onion rings hot with your favorite dipping sauces.