Heat the oven to 425°F and toss the cauliflower with the olive oil and 1/2 teaspoon salt in a 9-by-13-inch dish.
Roast the cauliflower until browned and nearly tender, 18 to 20 minutes.
Reduce the oven to 375°F and fold in the cooked chicken, bell pepper, green chiles, chili powder, cumin, salsa and remaining salt.
Top the mixture with pepper Jack and bake until bubbling, 12 to 15 minutes.
Rest the casserole for 5 minutes so the sauce clings to the cauliflower.
Scatter the crushed tortilla chips over individual servings immediately before eating.