In a large bowl, whisk the flour, granulated sugar, and salt to blend. Slowly pour in the milk, whisking constantly, until you have a smooth batter; don't worry about a few little lumps.

Line a baking sheet with paper towels and set it near the stove. Heat about 2 inches of oil in a medium deep pot or a deep-fryer; the pot should be tall enough that the oil won't overflow when you add the apples.

When the oil reaches 350°F, dip an apple ring in the batter until well coated and carefully slide it into the oil; repeat with 2 more slices. Fry the apple rings, turning once, until golden brown, about 3 minutes on each side. Transfer to the paper towel-lined sheet and continue frying the remaining apples, a few at a time. Scoop out any little bits of batter from the oil so they don't burn.

Stir the confectioners' sugar and cinnamon together in a small cup and generously dust the fried apples with the sugar and spice. Serve immediately.