I was going through my mom's old recipe box and found this. It took me a long time before I tried it but boy, it's the best batter I've found yet for onion rings.
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1 cup flour
2 tsp. baking powder
1 tsp. salt
1 egg separated
3/4 cup lukewarm water
1/4 cup beer
1 Tbsp. melted shortening
2 medium onions

Slice onions to desired thickness.  Soak in cold water for 10 minutes. Beat egg white until stiff.  Combine flour, baking powder and salt, add yolk of egg and beat with lukewarm water and melted shortening; fold in egg white.  Dip sliced onions in batter and deep fry at 375 for 3-5 minutes.  Drain on brown paper bag. Makes a nice thick, airy crust.