Make the Dipping Sauce: In a medium bowl, whisk the sour cream with the mayonnaise, horseradish, garlic powder, paprika, salt and pepper to combine. Set aside.
Make the Onions: Slice an onion in half vertically, but don’t cut all the way through to the base. The idea is to keep the bottom intact. Turn the onion 45 degrees, slice vertically again and repeat until you end up with what looks like little rectangles of onion, still attached at the base. Repeat with the remaining onions.
Loosen the onions a little by wiggling them around to separate the pieces slightly and create space between them. Place the buttermilk in a shallow dish and then add the onions to it. Toss well and massage the onions to get the buttermilk into the score marks.
In a medium-size shallow dish, whisk the flour with the salt, pepper, garlic powder, cayenne and baking powder to combine.
Heat 3 inches of oil in a wide pot over medium-high heat until it reads 350°F on a thermometer.
Working one at a time, remove an onion from the buttermilk and dredge fully in the flour mixture, working again to make sure the breading gets into the score marks.
Working in batches, fry the onions until they’re golden and crisp, flipping them over halfway through cooking, about 8 to 10 minutes. Set the cooked onions on paper towels to absorb the excess oil.