In a large non-reactive bowl (use glass or plastic), add 2 cups of milk and the juice of 1/2 a lemon. Stir and then let it sit for about 5 minutes. The milk will slightly curdle- you are making buttermilk here- or you can substitute with 2 cups of buttermilk if you have it.
Add 1 sliced sweet onion and stir to coat the onion in the milk. Allow it to soak for 20-30 minutes.
When the time is almost up for the onions to be done soaking, grab a large, shallow bowl. Add 2 cups of flour, 2 tsp salt, 1 tsp black pepper, and 1/2 tsp cayenne pepper (add more if you like heat). Stir the flour mixture and set aside.
Then grab a large cast iron pan or high-walled skillet (something good for frying). Fill the pan with enough oil so that it is about an inch and a half deep. Heat the oil over medium-high heat to about 350-365 degrees.
Using a pair of tongs, pull out about 1/3 of the onion slices, shaking off the excess milk, and place into the flour mixture. Coat the onions in the flour, again shaking off excess, and place into the hot oil.
Let the onions cook for 3-5 minutes, stirring occasionally so that they don’t stick together, until the onions are golden and crispy. Remove the fried onions to a plate lined with paper towels to absorb the excess oil. Give the onions a light sprinkle of salt.
When all of the first batch of onions has been removed to the plate, start on the next third of onions (you’ll be cooking 3 batches to not overcrowd).
Remove more of the onions from the milk, shaking off the excess, and place in the flour mixture. Coat the onions in the flour, shake off excess, and add to the oil. Make sure that the oil temperature has popped back up to 350-365.
Fry the onions the same as the steps above. Then fry the remaining onions as well.
Serve the onions on a salad, sandwich, or they are great on their own. Store leftovers in an air-tight container on the counter for up to 3 days. Enjoy!