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° for 40 minutes.)
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.
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.