Preheat oven to 400°F with rack in upper third position (about 5 inches from heat source).
Heat 2 tablespoons oil in a large skillet over medium-high heat.
Add cubed squash, sliced bell peppers and sliced onion in an even layer; cook, undisturbed, until starting to brown, 4 to 6 minutes.
Stir and continue to cook, stirring occasionally, until the onions have softened and the peppers and squash have started to become tender, 6 to 8 minutes.
Add ¼ cup water; cook, stirring to scrape up any browned bits, until the squash and peppers are just tender, 2 to 3 minutes.
Stir in 1½ tablespoons taco seasoning and ½ teaspoon salt until the vegetables are well coated.
Add 1¾ cups enchilada sauce, the rinsed beans, 1½ cups corn, the tortilla strips and the remaining 1 cup water; stir to incorporate.
Top evenly with 1 cup cheese.
Bake until the cheese has melted and the mixture is bubbling around the edges, about 10 minutes.
Increase oven temperature to broil; broil until the cheese has started to brown, 2 to 3 minutes.
Dollop with 3 tablespoons salsa.
Garnish with sour cream and cilantro, and serve with lime wedges, if desired.
