Preheat the oven to 375°F. Arrange squash in a large baking dish, cut-sides down. Pour 1/2 cup water into the dish and bake until just tender, 30 to 35 minutes. Rake with a fork to remove flesh in strands, leaving the shell intact for stuffing.
For the filling, heat oil in a large skillet over medium heat. Add onion, jalapeño and bell pepper and cook for 2 minutes or until soft. Add beans, corn and chili powder; cook, stirring frequently, 1 minute longer. Add cooked squash, cilantro, lime juice and salt, cook 1 minute until heated through.
Fill squash halves with filling, mounding mixture in the center.