Pour the oil to depth of 3 inches in a Dutch oven and place over medium-high heat. Bring to 375 degrees.
Cut the onions into ¼-inch slices and separate into rings. In a medium bowl, combine the flour and cornmeal. In a separate bowl, combine the egg, milk and club soda. Add to dry ingredients and stir until smooth. Stir in the jalapenos.
Dip the onion rings into the batter, coating well. Fry a few at a time until golden brown, around 3 to 4 minutes. Drain on paper towels, and sprinkle with salt. Serve immediately.
Recipe by The Tennessee Magazine at https://www.tnmagazine.org/flavor-from-the-underground/