Use a fork to shred the king oyster mushrooms lengthwise into thin, pulled strands. This will create a meaty texture.
Heat the oil in a large sauté pan over medium-high heat.
Add the shredded mushrooms to the hot pan and season with salt and pepper.
Sauté for 5-7 minutes, stirring occasionally, until the mushrooms have released their water and begin to brown and crisp on the edges.
Add the diced onion and sauté for another 2-3 minutes, or until translucent and softened.
Stir in the barbecue sauce.
Bring the mixture to a boil, then reduce heat to a simmer. Quickly remove the pan from the heat, add the Jack cheese, and stir until the cheese has completely melted into the mixture.
Place a warm tortilla in the middle of each warm serving plate. Spoon equal portions of the BBQ mushroom mixture into the middle of each tortilla.
Top with a small portion of Mango-Pickled Red Onion Salad, a sprinkle of the Cotija cheese and pumpkin seeds, and garnish with the serrano chiles and cilantro.