Crosscut onions into 1/2-inch rings, discarding onion hearts. In separate container combine eggs, milk, and beer. Mix flour and baking powder thoroughly in separate container. Soak onions in milk mixture and coat onions in flour. This step is repeated a second time, and may be done a third time to insure that the entire onion is dusted with flour mixture. Fry onions in a deep-fryer until golden brown.