This is the recipe I use. I use my mandolin to slice the onions.
2 large onions, halved, thinly sliced
1 cup buttermilk
2 cups all purpose flour
1 tablespoon salt
2 teaspoons paprika
1/4 teaspoon cayenne pepper
Vegetable oil (for frying)
Separate onions into strips. Mix with buttermilk in large bowl. Cover with plastic and refrigerate 1 hour. (Can be prepared 1 day ahead. Keep refrigerated.)
Stir flour, salt, paprika, and cayenne pepper in another large bowl. Drain onions. Add to flour mixture and toss to coat. Pour oil into heavy saucepan to depth of 3 inches. Heat to 350°F. Working in small batches, shake excess flour off onions and cook in hot oil until golden brown and crispy, about 2 minutes per batch. Using slotted spoon, transfer onions to paper towels and drain.
Transfer onions to platter. Season with salt.