In a large nonstick skillet, heat the vegetable oil over medium-high heat. Add the white and light green scallion parts, the butternut squash, shiitakes and garlic, and cook until the squash begins to soften, about 3 minutes. Stir in the chili powder and cook 30 seconds. Add 2 cups water and simmer until the vegetables are tender and a thick sauce forms, about 8 minutes.
Layer 2 tortillas together to make 12 separate stacks, then divide the mushroom mixture among the tortillas. Top with the dark green scallion parts and the jalapeños. Serve the tacos with the lime wedges.