Peel and cut an onion into 1/4 inch to 1/2 inch rings. Ensure to remove the thin clear membrane on the inside of the onion as this will shrink during frying, potentially tearing a hole in your batter.
Mix all your dry ingredients, and coat each onion ring in the flour mixture.
Set the onions aside and add your stout and ice to your flour mixture to make your batter.
Coat each onion ring in the batter, taking care to let the excess batter fall off.
Heat a pot of oil and use a thermometer to test once it has reached to 365°F.
Add your onion rings to the hot oil and fry for approximately 2-3 minutes or until evenly brown on both sides. Only do a few at a time to not crowd the pot.
Rest your fried onion rings on a wire rack to drain the excess oil.
Serve warm.