Softshell Crab Sandwich with Collard Slaw

Ingredients

• 1⁄2 cup mayonnaise
• 1 1⁄2 tsp. bread and butter pickles, finely chopped
• 1 1⁄2 tsp. fresh lemon juice
• 1 1⁄2 tsp. minced sweet onion, such as Vidalia
• 1⁄2 tsp. garlic powder
• 1⁄2 tsp. chile flakes
• 1⁄2 cup chile sauce
• 1⁄2 cup ketchup
• 3 tbsp. prepared horseradish
• 1 tbsp. fresh lemon juice
• 2 tsp. Asian hot sauce, such as Sriracha
• 1⁄2 cup apple cider vinegar
• 3 tbsp. sugar
• 2 tbsp. canola oil
• 1⁄2 tsp. celery seed
• 1⁄8 tsp. dry mustard
• 1⁄2 small head green cabbage, shredded
• 2 tbsp. minced parsley
• 6 leaves collard greens, stemmed and thinly sliced
• 1 medium carrot, peeled and coarsely grated
• 1⁄2 small onion, coarsely grated
• Kosher salt
• freshly ground black pepper
• 8 softshell crabs, cleaned
• 2 cups flour
• 1⁄3 cup olive oil
• 3 tbsp. unsalted butter
• 4 buns, toasted