Thinly slice onions into crescents, then gently pat them dry and place them in a bowl. Mix milk and vinegar, then pour over sliced onions. Soak for 1 hour.
In another bowl, mix together flour, cornstarch, smoked salt, garlic powder, smoked paprika, and chipotle powder. Dredge onions in the flour mixture until evenly coated.
Heat oil in a medium-sized pot to 350°F, then fry battered onions, in batches if necessary, for 1-2 minutes or until golden brown. Remove from the oil using tongs or a slotted spoon, then strain over a wire rack or colander. Sprinkle with additional smoked paprika, smoked salt, and garlic powder before serving.
Notes
Don't be alarmed if the milk starts to curdle. Like buttermilk, this mixture is slightly acidic, similar to kefir or yogurt, which is more effective for tenderizing the onions than milk alone.
Oil temperature is important! If it's too hot, the onions will burn before the batter can set, and if it's not hot enough, the soggy batter will slip off. Try to keep it between 350-375°F.
Patting the excess juice from the onions helps ensure that the batter will adhere to the onion slices.
I prefer a more rustic cut, but if you want the onions sliced extra finely, you can use a mandolin.
Allowing excess oil to drain using a strainer or wire rack is preferable to a paper towel, which will harbor excess moisture, softening the frizzled onions.
You can flip the onions halfway through frying, but don't touch them before they've sat for at least 30-45 seconds, as this can cause the batter to detach.
Store leftovers in an airtight container lined with a paper towel and keep in the fridge or a cool, dark pantry for up to 3-4 days.
Don't store frizzled onions in a plastic bag, as this will cause them to soften.
Make sure they're completely cool before storing to ensure they don't trap excess moisture.