To make the crab burgers:
In a large bowl, whisk mayonnaise, egg, scallions, Worcestershire sauce, dry mustard, hot sauce, and salt and pepper, to taste. Fold in the crabmeat and 1 cup of the bread crumbs until just combined. Line a baking sheet with parchment paper. Divide the mixture into 4 equal portions and form into patties. Put the patties on the baking sheet, cover with plastic wrap and refrigerate for at least 30 minutes to firm up.
Meanwhile make the slaw: In a small bowl, whisk the mayonnaise, sour cream, vinegar, Dijon, lemon juice, paprika, cayenne, parsley and salt and pepper, taste. Using the grater attachment on the food processor or a box grater, grate the celery root. Work quickly so that the root doesn't turn brown. Transfer to a medium bowl, add pour the dressing over the celery root. Toss to combine and refrigerate for at least 10 minutes before serving.
Remove the crab patties from the refrigerator. Place the remaining bread crumbs in a shallow dish. Dredge the patties in the bread crumbs and pat along the edges.
Heat the oil in a 12-inch cast iron skillet over medium-high heat until the oil is hot but not smoking. Place the patties into the oil and cook about 3 minutes per side. Transfer to paper towel lined plate to drain. Season the patties with salt while they are still warm. Place on baking sheet to hold in a preheated 250 degree oven while you toast the buns.
Slice the kaiser rolls in half, butter, put on a sheet pan and toast under the broiler.
To assemble the burgers, place a patty on the bottom half of each bun and top with the Celery Root Slaw, a piece of lettuce and a tomato slice. Cover with the other half of the bun and serve with Rajun' Cajun Onion Rings.
In a large bowl, whisk together the sour cream, the milk and the hot sauce. Add the onion rings and set aside while you season the flour. In a large baking dish or pie plate, combine the celery salt, onion powder, garlic powder, mustard powder, paprika, chili powder, white pepper, salt and the flour.
Heat about 3 inches of oil to 350 degrees F in a large Dutch oven.
Remove the onion rings from the liquid mixture and dredge in the flour mixture. Repeat the process to double coat the rings. Place the onion rings into the oil in batches and fry until golden brown and crispy, about 5 minutes. Drain the onion rings on a paper towel lined plate and season with salt while still hot, then transfer to a serving platter.