Cut onions into 1/2-in. slices; separate into rings. In a shallow bowl, whisk egg and buttermilk until blended. In another shallow bowl, combine the flour, cornmeal, pecans, salt and pepper. Dip onion rings in egg mixture, then coat with flour mixture.
In an electric skillet or deep-fat fryer heat 1 in. of oil to 375°. Fry onion rings, a few at a time, for 1 to 1-1/2 minutes on each side or until golden brown. Drain on paper towels.