Holding the oyster mushrooms by their thick stalk, pull it apart into long strips. Keep doing this until you have a pile of thin strands.
Heat a medium skillet (frying pan) on medium high for 1 to 2 minutes. When hot, drizzle with 1 to 2 tablespoons of oil and, working in batches, add the mushrooms. Toss to coat in the oil and then leave to cook undisturbed for 1 minute, and then toss again. Keep doing this until the mushrooms are dry and golden with crispy edges, 7 to 8 minutes. Cook the remaining mushrooms. (Alternatively, toss the mushrooms with olive oil and roast in a single layer at 400˚F / 200˚C until golden, 15 to 20 minutes)
Transfer the mushrooms to a bowl and add the smoked paprika, garlic powder, onion powder, 3/4 teaspoon salt, 2 teaspoons olive oil, and the juice of 1/2 lime and toss to coat.
Meanwhile, heat the oven to 375˚F / 190˚C. Line a baking sheet with a wire rack.
Brush both sides of the tortilla with olive oil. Lay the tortillas out onto a baking sheet, as many as will fit, in a single layer (don’t overlap) on top of the wire rack and bake until golden and crisp, 14 to 15 minutes. (If you don’t have a wire rack, place them onto a regular baking sheet and flip them over half way)
Meanwhile, place the black beans, garlic, cumin, coriander, smoked paprika and turmeric into a small blender or food processor and blend until it looks like a puree. I prefer it a bit chunky so I leave some beans unblended. Season with sea salt and black pepper.
To serve, spread a layer of black beans onto a tostada. Top with mushrooms, the cheese, the cilantro/coriander leaves, and any of the optional toppings. Eat immediately.