Position rack in bottom of oven; preheat to 425°F. Arrange cauliflower, onion, bell pepper and poblano on a large rimmed baking sheet. Drizzle with oil and sprinkle with taco seasoning and salt; toss to coat. Spread into an even layer. Roast, stirring once, until the vegetables are browned and crispy around the edges, about 30 minutes. Remove from the oven.
Reduce oven temperature to 350°F. Lightly coat a 3-quart baking dish with cooking spray.
Transfer the roasted vegetables to a large bowl. Stir in beans, rice, corn, salsa and ½ cup cheese. Transfer the mixture to the prepared baking dish. Sprinkle with tortilla chips. Sprinkle the remaining 1 cup cheese over the top. Cover loosely with foil and bake for 20 minutes. Remove the foil and continue baking until the casserole is hot throughout and the cheese is melted and browned, about 10 minutes more. Garnish with cilantro, jalapeños, sour cream and/or lime wedges, if desired.
