Peel the onions, then slice them into 1/2-inch rings. Separate each ring out, then drop them in a bowl of water.
Place a large bowl on the counter with the flour in it, then the melted vanilla ice cream, then the cornmeal. Finally, place an empty cookie sheet next to your fryer.
Preheat your deep fryer to 350 degrees. Remove an onion ring from the bowl, shaking off some of the water, then dip it into the flour, then the melted ice cream, then the cornmeal. Place it on the cookie sheet to dry. Repeat with the balance of the onions, then let them air dry a bit. About 5-10 minutes.
Drop a portion of the onion rings into your fryer basket, ensuring that they don't crowd each other too much, then fry in batches until golden brown. About 3-4 minutes per batch.
Serve with plenty of ketchup.