1. In a medium mixing bowl, toss julienned radishes, turnips, and carrots with sea salt.
2. Let sit at room temperature for 30 minutes.
3. Pour off any accumulated juices. Fold in green onions, buttermilk, mayo, and celery seed. Chill in fridge while fish cooks. Before serving, add sea salt to taste.Rinse fish and pat dry. In a wide mixing bowl, stir to combine buttermilk, sea salt, and Old Bay.
4. Pour panko into a deep-sided plate or shallow bowl. Line a small rimmed baking sheet with paper towels. Preheat oven to 200 degrees F.
5. Heat 1-inch canola oil in a heavy-bottomed skillet over medium-high heat. Working in batches with tongs, dip fish in buttermilk, allow excess to drip off, and gently press into panko on both sides. Carefully set fish in hot oil and fry until golden, 2 - 3 minutes per side. Set fish on prepared baking sheet and keep warm in the oven.
6. Serve fish with toasted potato slider buns and plenty of bread and butter pickles.