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.
Combine the cornmeal, flour, dry mustard and salt in a blender and blend at medium speed 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, 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. Continue until all strings are fried and serve hot with Legal Seafoods Chipotle or House Tartar Sauce.