2 small yellow onions, halved lengthwise and thinly sliced crosswise
2 cups buttermilk
2 cups all-purpose flour
Salt and freshly ground pepper
Vegetable oil, for frying
4 medium heads of iceberg lettuce, each cut through the core into 6 wedges
In a medium bowl, whisk the mayonnaise with the buttermilk, ketchup, sweet pickle relish and Worcestershire sauce. Season with the salt, pepper and cayenne and refrigerate until chilled.
In a medium bowl, cover the onions with the buttermilk and let stand for 10 minutes. In a shallow dish, generously season the flour with salt and pepper.
In a medium cast-iron skillet, heat 2 inches of vegetable oil to 350?. Drain the onions in a colander, then dredge them in the seasoned flour. Transfer the onions to a strainer and shake off the excess flour. Working in 3 to 4 batches, fry the onions in the hot oil until golden brown, about 2 minutes per batch. Using a wire skimmer, transfer the fried onions to paper towels to drain. Season with salt.
Arrange the iceberg wedges on 2 large platters. Top each wedge with a generous tablespoon of the dressing and crispy onions and serve immediately.