1. Pierce squash (about 1 inch deep) all over with a knife. Place in a microwave-safe dish and microwave on HIGH for 7 minutes. Turn squash over and microwave another 8 to 10 minutes, until slightly soft. Cool 5 minutes. (Alternatively, you can roast squash. Cut in half lengthwise, scoop out seeds and roast cut side down in an oiled baking pan at 375 degrees for 40 minutes.)
2. Scoop squash flesh into a bowl, discarding seeds and skin. In a small bowl, whisk lime juice, chili powder, cumin, coriander and salt. Pour over squash and gently toss.
3. Heat a skillet over mediumhigh heat. Warm and slightly blister tortillas, about 30 seconds per side. Transfer to a platter and top each tortilla with 2 tbsp each beans and squash and 2 tsp cheese. Garnish with onion and cilantro and, if using, hot sauce and lime wedges.