For the spice mix: Combine cayenne, garlic powder, onion powder, smoked paprika, black pepper, salt and sugar in a bowl. (This will keep for 3 to 4 months in an airtight container.)
For the slaw: Combine the cabbage and Pickled Celery thoroughly.
For the scallion mayo: Add soy milk, lemon juice, mustard, salt, vinegar, scallions and garlic to a blender and blend until a homogenous color. With the blender running, slowly add the oil to emulsify. If the mayo becomes too thick, add a little ice water to thin it out.
For the sandwich: Heat oil in a deep-fryer to 350 degrees F.
Heat cornstarch in 4 quarts water in a saucepan until clear.
Coat mushrooms in cornstarch slurry, then fry until the exterior is golden brown. While the mushroom is still very hot from the fryer, coat in 1 tablespoon of the spice mixture, and a little extra salt. Coat both sides of the focaccia in the scallion mayo, then place the mushroom on top, and 1 cup slaw on top of that.
Bread-and-Butter Pickled Celery:
Heat up the sugar, vinegar, mustard seeds, salt, turmeric, celery seeds and coriander seeds in a pot until it reaches a boil. Pour over the celery and onion in a large bowl. Let sit until cool.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.