- Peel the onions and cut a pie-shaped wedge from the top to the bottom of each onion. Place the onion sideways on a counter and hold it tightly with one hand, almost squeezing the onion closed. Thinly slice the onion and separate out each strand. Continue until all onions are sliced.
- Cover the onion strands with the buttermilk and let soak for 15 minutes.
- Meanwhile, combine the cornmeal, flour, dry mustard and salt in a blender or food processor and pulse until thoroughly combined, about 20-30 seconds.
- Add the oil to a deep fryer or a heavy, deep, nonreactive pan. Heat to 360°F. While the oil is heating, drain the onion strands and toss them in the cornmeal flour coating. It is easiest to do this in a ziplock bag in small batches and shake, but you can also do this in a bowl.
- When the oil is up to temperature, place a small batch of strings into the oil and fry them until they are lightly browned and crisp, about 30 seconds per batch.
- Remove with a slotted spoon or fry basket, draining excess oil, and place on a baking sheet lined with paper towels and sprinkle with additional salt to taste, if desired. Continue until all strings are fried and serve hot with Sriracha Horseradish Cream Sauce.