Line a large plate with paper towels and set aside. Pour oil into a cast iron pot and heat up to 375F in oven while you prepare the ingredients (if you're using cast iron, if not then skip this step and heat oil in med. frying pan over med. heat).
Peel and slice off the top and bottom from the onion. Thinly slice to about 1/8th inch round slices. Mix the salt and pepper into the flour until well combined and pour the milk into a small bowl.
Dip the onion slices into the milk, making sure they are completely covered in the milk. Next dredge them in the flour also making sure they’re throughly coated.
Carefully place the onion slices into the hot oil and fry until golden brown on both sides, this will take about 2 to 3 minutes on each side. Remove the onion slices and drain off excess oil on paper towel lined plate.
Allow to cool before using. Chop into small pieces if making copycat French's fried onions to top on to green bean casserole etc... Any leftovers may be stored in an airtight container and in the refrigerator.