Preheat the oven to 400°F (200°C, or gas mark 6), and line a baking sheet with parchment paper or a silicone mat.
Place the butternut squash, mushrooms, black beans, and yellow onions in a mixing bowl. Drizzle with oil and toss to coat, then add the chili powder, cumin, salt, oregano, paprika, and cayenne pepper, and toss again. Spread the mixture out on the baking sheet, leaving some space for the tomatillos.
Place the tomatillos on the remainder of the baking sheet, then place in the oven for 15 to 20 minutes, or until the squash is fork tender. Sprinkle with more salt to taste, if desired.
Warm the tortillas in either the microwave or on a hot pan until soft and pliable. Fill each one with some butternut– black bean mixture, then top with tomatillos, shredded cabbage, and jalapeño slices. Serve warm, accompanied by lime wedges.