Prepare the vegan buttermilk by whisking together the soy milk, apple cider vinegar, hot sauce, and Dijon mustard in a medium bowl. Let it sit for 5-10 minutes to curdle and thicken slightly.

In a separate wide bowl or baking dish, whisk together the all-purpose flour, cornstarch, smoked paprika, garlic powder, onion powder, poultry seasoning, cayenne pepper, kosher salt, and black pepper.

Heat the neutral frying oil in a heavy-bottomed pot or Dutch oven over medium-high heat until it reaches 350°F (175°C). Line a wire rack over a baking sheet.

Dip each oyster mushroom cluster first into the wet batter, letting the excess drip off, then coat thoroughly in the dry dredge, pressing the flour mixture gently onto the mushroom so it adheres. For extra crunch, repeat the process (double dip).

Carefully lower the breaded mushrooms into the hot oil in batches, being careful not to overcrowd the pot. Fry for 4-5 minutes per side, or until deeply golden brown and crispy. Transfer to the prepared wire rack.

While the last batch is frying, begin the gravy. Melt the vegan butter in a large skillet over medium heat. Add the sliced onions and sauté until softened and golden brown, about 8-10 minutes.

Sprinkle the 1/4 cup of flour over the onions and stir well to create a roux. Cook for 2-3 minutes, stirring constantly to lightly toast the flour.

Gradually whisk in the warm vegetable broth to prevent lumps. Add the soy sauce, dried thyme, and black pepper. Bring the gravy to a simmer, then reduce the heat to low. Let it cook until it thickens to your desired consistency, about 5-7 minutes. Taste and adjust salt if necessary.

To serve, place the crispy fried oyster mushrooms on a plate and generously ladle the hot onion gravy over the top, or place the fried mushrooms directly into the skillet with the gravy right before serving if you prefer them slightly softened by the sauce.