Wipe the oyster mushrooms clean with a damp cloth, then set them aside. In a shallow, wide bowl, add vegan buttermilk, flour, onion powder, garlic powder, Cajun seasoning, black pepper, and salt. Whisk until fully incorporated.
In another shallow bowl, add all-purpose flour, cornstarch, baking powder, Cajun seasoning, black pepper, and salt. Whisk together until well combined.
Heat a deep pot of canola, peanut, or other high-heat oil until it reaches a temperature of 350-375F. While the oil is heating, dredge each piece of mushroom in the dry coating, then dip it in the buttermilk batter, shaking off any excess. Return the mushroom to the dry coating for a second layer, again shaking off any excess flour.
Carefully add the coated mushrooms to the hot oil, ensuring not to overcrowd the pot. Fry each piece for about 3-4 minutes, or until golden brown and crispy.
Use a slotted spoon to remove the mushrooms from the oil and transfer them to a wire rack set over a baking tray to drain any excess oil.
Serve your vegan fried oyster mushrooms while they’re warm on their own, with your favorite dipping sauce, or added to a sandwich or burger.