For the charred scallion mayo: Combine the Donkey Sauce and scallions in a small bowl and refrigerate until ready to serve.
For the fried oyster mushrooms: Heat 3 to 4 inches of oil in a Dutch oven to 375 degrees F. Carefully dredge the mushroom in flour ensuring it gets in between the stems. Shake off excess.
Mix the flours together in a large bowl and make a well in the center. Add the eggs to the center and whisk to combine completely. Whisk in the seltzer water. Season with salt. Dip the mushrooms into the batter and then into the fryer. Fry, turning halfway through and holding under the oil, until golden, about 4 minutes. Remove to a wire rack.
For the burgers: Brush the cut sides of the ciabatta with the garlic butter and toast until golden brown. Smear both sides with the charred scallion mayo. Combine the cabbages with the rice vinegar, olive oil, chiles, charred scallions, garlic, mustard, salt and pepper in a medium bowl and mix until tender. Place a fried mushroom on the base bun, top with half the dressed slaw, hit with juice of half the lemon and place the top half of bun on top. Repeat with the other burger.
Combine the mayonnaise, roasted garlic puree, mustard, salt, pepper and Worcestershire sauce in a medium bowl. Refrigerate for 30 minutes before using.