The batter's spicy sweetness stems from the unexpected--sweetened condensed milk.
Cut onions into 1/4-inch-thick slices, and separate into rings.
Combine flour and cornmeal in a bowl. Combine egg, condensed milk, and club soda; add to dry ingredients, stirring until smooth. Stir in peppers.
Pour oil to depth of 3 inches into a Dutch oven; heat to 375°.
Dip onion rings in batter, coating well; fry, a few rings at a time, until golden. Drain on paper towels, and sprinkle with salt. Serve immediately.