I like onion rings thin and crispy.  I read an article by a chef that
said the secret to making them crispy at home is to soak them in
buttermilk.  This recipe proves he was right.

Restaurant-Style Crispy Onion Rings

At restaurants, onions can be sliced very thin on a meat slicer.  By
cutting the onions in half first, home cooks can get close to the same
thinness with a knife, though it means sacrificing the ring shape.

2               large onions, thinly sliced
1      cup   buttermilk

2      cups all-purpose flour
1      Tbsp salt
2      tsp    paprika
1/4  tsp    cayenne pepper
                 vegetable oil for frying

Separate onions into strips or rings.  Mix with buttermilk in a large
bowl.  Cover with plastic and refrigerate 1 hour.  Can be prepared 1
day ahead if kept refrigerated.

Stir flour, salt, paprika and cayenne in another large bowl.  Drain
onions and discard buttermilk.  Add onions to flour mixture and toss
to coat.  Pour oil into a heavy saucepan to a depth of 3 inches.  Heat
oil to 350 degrees F.  Working in small batches, shake excess flour
off of onions and cook in hot oil until golden brown and crispy, about
2 minutes per batch.  Using slotted spoon, transfer onions to paper
towels to drain.