Cut the onions in 1/4" slices. In a glass dish, soak the onions in the buttermilk for 30 to 60 minutes.
In a brown paper sack, combine the flour, cornmeal, salt, onion powder, chili powder, and if needed, sugar. Drain the onions lightly and dredge them in the seasoned flour.
Pour at least 4" of oil into a heavy saucepan. Heat the oil to 375°. Fry the onions, in batches, for 2 to 3 minutes, or until golden. For the crispiest results, drain on paper towels and spread on a serving plattter rather than piling them into a basket, where they could become soggy. Serve immediately.