In a large bowl, combine egg, 1/4 cup oil and milk. Mix with an electric mixer, set to low speed, for 1 minute. Add dry ingredients and continue to mix until smooth.
Add 6 cups oil to a large, heavy bottomed pot set to medium heat. Preheat for about 10 minutes, or until oil reaches about 375 degrees. Reduce heat to low-medium. Prepare a large baking sheet with paper towels, to drain excess grease off fried onion rings.
Dip onion slices in batter, then place into hot oil. Do this in batches of 4-5 onion rings at a time, so they don't stick together. Fry until golden on both sides, flipping after 30 seconds to 1 minute. Be attentive and take care not to burn. You may need to continue to reduce your heat as they cook.
With a slotted spoon, remove onion rings to paper towel lined sheet. Repeat with remaining batches.
Serve with ketchup, or a spicy ketchup + Sriracha mixture.
This recipe can be easily doubled for a crowd. You can cook them a few hours ahead, and keep warm in your oven before serving.Adapted from Genius Kitchen