Whisk together the buttermilk, sour cream, dill, garlic powder, onion powder and cayenne pepper in a bowl. Store in an airtight container in the refrigerator.
Heat an inch of oil in a heavy bottom skillet over medium heat.
Whisk the eggs and milk in a shallow bowl or pie plate. Set aside. In another shallow bowl, whisk together the flour and dry ranch seasoning mix. Grab a large plate to place the dipped onion rings.
Dredge the onions through the flour mixture with one hand. With the other hand, dip them in the egg mixture. Drip off excess egg. Add back to the flour mixture using dry hand and toss until coated. Place on the clean plate.
Fry the onion rings in a single layer in batches for about 2 to 3 minutes. The rings should submerge in oil but if they don't, flip them to brown the other side. Remove to a paper towel lined plate or cooling rack.