01 - Trim the root end just enough so each onion stands upright while staying intact. Peel away the outer skin. Cut 4 to 6 vertical slices from stem to root, stopping about ¼ inch from the base to keep the layers connected. Rotate and add more cuts to form 8 to 12 evenly spaced petals. Gently loosen the petals with your fingers.
02 - Combine the flour, cornstarch, smoked paprika, garlic powder, onion powder, salt, black pepper, and cayenne in a shallow bowl. In a second bowl, whisk together the eggs and whole milk until uniform.
03 - Dredge each scored onion in the flour mixture, ensuring all surfaces and petals are well coated. Shake off the excess, then submerge in the egg-milk wash. Return to the flour mixture and press gently to adhere the coating to every petal.
04 - Pour the vegetable oil into a deep pot or fryer and bring to 350°F over medium-high heat. Monitor with a thermometer for accuracy.
05 - Lower one or two onions into the oil cut-side down. Fry for 2 to 3 minutes until the bottom is golden, then flip and continue frying for another 2 to 3 minutes until crisp and cooked through.
06 - Lift the onions from the oil with a slotted spoon and set on paper towels to drain. Season with a light sprinkle of salt while still hot.
07 - Combine the mayonnaise, ketchup, prepared horseradish, smoked paprika, garlic powder, lemon juice, and a pinch of salt in a bowl. Stir until fully incorporated.
08 - Arrange the hot mini bloomin onions on a serving platter alongside the dipping sauce for immediate enjoyment.