Slice your apples (unpeeled) into rings about 1/4" thick and cut out the cores from the middle, making a ring. In a medium bowl, whisk together the flour, cornstarch, baking soda and enough milk or water to make a thin batter – it should have the consistency of thin cream.
In a wide pot, heat an inch or two of canola oil over medium-high heat until hot but not smoking. (You’ll know it’s ready when a scrap of bread held into the oil bubbles around it.) Dip a few slices of apple at a time into the batter, coating them completely, then gently slip into the oil. Cook a couple at a time for a minute or two, flipping with tongs as necessary, until golden. (If they are cooking too quickly, turn the heat down – if it’s taking too long, the oil may need to be hotter. Don’t crowd the pot, or it will bring the temperature of the oil down.)
Remove with a slotted spoon and transfer to paper towels to drain. Once cool enough to handle and eat, sprinkle lightly with powdered sugar.