I do beer battered onion rings on the food truck. Make a batch almost daily.
My method is to dip and par fry in 300* oil in the open kettle, no basket just until the batter is set, remove to basket to drain, cool on sheet pan and store in fridge, keeps well for about 24 hrs.
Fried to order at 350* to finish. They are very crisp and the onion is cooked through.
My batter uses 2lbs cornstarch to three 16oz cans beer with three eggs, baking powder, flour, s&p, granulated...