Place sliced onion in a wide shallow bowl, and pour coconut milk and apple cider vinegar over it.
Toss a few times to coat, then let it sit for 10 minutes.
Prepare a separate bowl with tapioca starch, coconut flour, and sea salt. Mix to combine.
Pour your choice of cooking oil in a large skillet until the oil is about 1 inch deep, and heat over medium high heat. Prepare a large dish with 2-3 layers of paper towels.
Use a tong or a fork to transfer a handful of onions from the coconut milk, then to the flour mixture.
Coat and dust off excess flour, then fry in the skillet for 30 seconds on each side, or until crispy and golden brown.
Transfer to the paper towel lined dish. Repeat this process until you fry all the onions, adding more oil to the skillet as needed.
Use right away on casseroles or eat them on their own with a side of ketchup!