Peel and slice the yellow onions into ¾” thick slices and separate into rings. If you prefer your onion rings thinner or thicker, slice it to your preference. Keep in mind that thinner rings will cook faster and thicker rings will take longer to cook.
Place the sourdough starter discard in a large bowl. In a separate dish, place the flour, garlic powder, pepper, paprika, and cayenne pepper. Stir together until well-blended.
Pour enough oil into a deep Dutch oven, about 1½”. Place a wire rack on a rimmed baking sheet and set aside.
Heat the oil until it reaches 375°F. If you don’t have a candy thermometer, place a 1” cube of bread in the oil. The cube should turn golden brown in about 60 seconds.
Once the oil has reached temperature, dip an onion ring in the sourdough discard and coat. Move the ring with a fork to the flour mixture and toss to coat. Do this with three or four onion rings before placing in oil. As the flour mixture starts to clump, rub clumps between your fingers to break them into smaller pieces.
Fry the rings in the oil by placing a ring partially in the oil, releasing from your fork. Don’t overcrowd the pan. Fry the rings for about 1½ to 2 minutes, then flip over with tongs and continue to fry until golden brown. Move the fried onion rings to the wire rack and repeat with the remaining onion rings.
Once all the onion rings are fried, sprinkle with extra salt and serve warm with your favorite condiment.