Peel and thinly slice the onion. Use a mandolin to get uniformly thin slices. You can either leave the onion rings whole or cut them in half to create strings. Transfer the onions to a bowl and separate the layers. Pour the buttermilk over the onions, pressing down to fully submerge. Let sit while the oil preheats.
Add 2 inches of vegetable oil to a deep, medium saucepan. Place over medium high heat and bring up to 350°F. Meanwhile, combine the flour, seasonings, salt, and pepper in a shallow dish. Stir to combine.
Line a baking sheet with a rack or paper towel and set aside. When ready to fry, lift a third of the onion slices out of the buttermilk, shaking off the excess. Add to the flour and toss to evenly coat. use your fingers break the onion strings apart and shake off any excess flour.
Carefully add the coated onion strings to the hot oil - the oil will bubble up once the onions are added. Use a slotted spoon to stir the onion strings occasionally. Fry the onions for about 5 to 7 minutes until the onion strings are golden brown.
Use a slotted spoon to transfer the crispy onions to the prepared baking sheet to drain. Sprinkle with kosher salt. Repeat two more times. The onions can be made ahead of time and will keep in an air tight container for 2 to 3 days at room temperature.
Buttermilk Mashed Potatoes
Peel the potatoes and cut into chunks. Add to a large pot of water and place over high heat. Bring the water to a boil and cook until the potatoes are fork tender. Drain the potatoes and transfer back to the pot.
Add the butter and begin to mash the potatoes. Add the buttermilk in two or three batches, mashing in between each addition. Season with the kosher salt and pepper. Mash until the potatoes are smooth, adding more buttermilk if you prefer a softer mash.
Transfer the mashed potatoes to a serving bowl. Dot with extra butter if desired. Pile the crispy onions on top and enjoy!