For the onion blossoms: Slice off the tip of each onion and gently peel and remove the outer skin. Turn an onion root-side down and make 4 vertical cuts with a paring knife at 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock, leaving the base intact to create petals. Repeat this process for each onion.
Add about 3 inches of oil to a large heavy-bottomed pot. Heat the oil to 350 degrees F over medium heat.
Whisk together the flour, garlic powder, oregano, smoked paprika, seasoning salt and cayenne in a large bowl or shallow dish. Combine the buttermilk and egg in another large bowl.
Place the onions in the flour mixture with a spoon, tossing slightly until they are well coated; shake off any excess. Submerge the onions cut-side up in the buttermilk mixture. Drain the onions slightly, then dredge them back in the flour mixture, making sure to coat each petal.
Carefully place each onion root-side down into the hot oil with a slotted spoon and fry, flipping once, until golden brown, about 2 minutes. Drain on a paper towel-lined plate or cooling rack. Repeat with the remaining onions. Sprinkle with salt and serve warm with the dipping sauce.
For the dipping sauce: Mix the mayonnaise, sour cream, ketchup, horseradish, cayenne, pepper and salt in a small bowl.