Combine the onions, vinegar, salt and pepper in a medium sized mixing blow and let sit for 30 minutes.
Heat oil to 350 degrees in a 8-quart heavy duty sauce pot, or a large cast iron skillet.
Whisk together the milk and egg in a mixing bowl. Place onions in the milk mixture then drain well.
Place the seasoned flour in another mixing bowl, and toss onions in the flour, making sure they are coated well. Remove onions from the bowl and shake off any excess flour.
Place half of the coated onions in the hot oil. Using a slotted spoon, gently turn 2-3 times. Fry 3-4 minutes. Remove onions and place on paper towel lined baking sheet to drain.
Fry the remaining onions and serve immediately.
Yield: 6-8 servings
A mandolin or meat slicer works best when slicing the onions.