Buttermilk Onion Rings

Name: Buttermilk Onion Rings
Prep time: 20 min
Total time: 20 min
Cuisine: American
Ingredients: Vegetable oil; 1 c. all-purpose flour; 1/3 c. fine cornmeal; 3/4 c. cornstarch; 1 tsp. baking powder; 1 tsp. freshly ground black pepper; 1/4 tsp. smoked paprika; 1 egg, lightly beaten; 3/4 c. buttermilk; 1/4 c. seltzer; 2 large Vidalia onions, cut into 1/2"-thick rings; Kosher salt, for sprinkling; Ketchup, for serving; Mayonnaise, for serving