For extra crispy onion rings, try double-dipping them in the batter before frying. If the batter becomes too thick, whisk in a little more buttermilk to thin it out. Don't overcrowd the pot when frying, as this will lower the oil temperature and result in soggy onion rings. For a spicier kick, add a pinch of cayenne pepper to the batter. These onion rings are best served immediately. However, if you need to keep them warm, place them in a preheated oven at 200°F (95°C) on a wire rack.