Whisk all ingredients together in a small bowl and keep chilled until ready to use.
Kohlrabi Crab Cakes
Trim ends off kohlrabi and peel with a vegetable peeler. Grate using a large grater and add to a separate bowl. Sprinkle with salt and allow to sit for 5 minutes. Drain and squeeze dry with a paper towel. Using the same large grater, peel and grate potato and pat dry with a paper towel. Add to kohlrabi.
In a large bowl stir together egg, dill, dijon mustard, celery salt and pepper. Stir in drained crab meat, prepped and dried grated kohlrabi and potato, bread crumbs, flour and green onions.
Add about 1 tbsp of oil to a large non stick skillet and heat over medium heat until hot. Form approximately 1/4 cup of batter into a 3-4" round patty. Flatten as much as you can while still retaining its shape. Pan fry for 5-6 minutes per side, adjusting heat if necessary. Remove to a paper towel lined plate and keep warm in a low oven while cooking any remaining cakes. Serve immediately with dill sauce.