In a medium bowl, whisk together the flaxseeds and 3 tablespoons of the water with a fork to combine.
Add the rice flour, arrowroot powder, cornmeal, salt, and black pepper to the flax mixture and stir to combine. Stir in the remaining 1 1/2 cups water and mix well.
Fill a Dutch oven or wok with safflower oil to a depth of about 2 inches and heat the oil to 350°F over medium-high heat. Heat the oven to 200°F. Line a platter with paper towels.
When the oil is hot, in three or four batches, dip the mushrooms in the batter and toss to coat them well. Carefully transfer them to the hot oil and fry, until crisp and golden brown, 4 to 5 minutes. (If needed, gently agitate them with a slotted spoon to break apart any mushrooms that begin to stick together.) Use a slotted spoon or a spatula to trans¬fer the mushrooms to the platter, season lightly with salt, then place in the warm oven. Repeat with the remaining mushrooms, adding more water to the batter, as needed, if it becomes too thick to coat.
While the mushrooms are frying, halve the bread horizontally, then crosswise, and lightly toast it. Spread the cut sides of the bread generously with rémoulade (I’m talking about a messy slather) and top the bottom halves evenly with the coleslaw. Add a generous handful of the mushrooms, sprinkle with pepper and douse with hot sauce. Cover with the top halves of the bread, and enjoy.