Remoulade sauce: Whisk together the mayonnaise, pickles, mustard, ketchup, hot sauce, onion powder, ¼ teaspoon salt and several grinds of pepper in a small bowl until combined. Cover and refrigerate until ready to serve.
Po Boys: Trim the mushrooms into bite-size pieces (roughly 2 inches) Oyster mushrooms can be sliced/torn lengthwise, while shiitake and cremini mushrooms are best quartered.
Pour the buttermilk and hot sauce into a shallow baking dish and whisk to combine. Add the mushrooms, the toss until evenly coated. Marinate at room temperature until well-seasoned, about 10 minutes.
Whisk together the flour, smoked paprika, 1 teaspoon salt and several grinds of pepper in a large bowl. Stir in 2 tablespoons of the buttermilk marinade to make shaggy clumps.
Heat 2 inches of vegetable or canola oil in a large pot over medium heat. Oil should reach 350 degrees F. Line a rimmed baking sheet with paper towels for draining.
Dip the mushrooms first in the flour mixture, turning to coat, them back in the marinade. Then dip again in the flour to coat with the clumpy bits, transfer to plate. Work in batches, fry the mushrooms, flipping occasionally, until golden brown and crisp, 2-3 minutes. Use a slotted spoon to transfer the mushrooms to the lined baking sheet and season with a pinch of salt.
For toasting the rolls we will be using butter and garlic mixture. 2 tablespoons of butter one tablespoon of minced garlic. Mix the together and spread on both sides of the inside of the bread. Place the bread face down on pan or skillet until golden brown.
Assemble sandwich: Assemble with tomatoes, lettuce, mushrooms, and drizzle sauce over the mushrooms.