Broil poblanos until blackened, steam in a covered bowl for 10 minutes, then peel, seed and cut into strips. Heat the oven to 400°F.
Toss cauliflower with oil and 1/2 teaspoon salt and roast 18 minutes. Grease a 9-by-13-inch dish.
Brown beef and onion and drain. Add cumin, chili powder and remaining salt for 30 seconds, then tomato sauce and simmer 4 minutes.
Fold in cauliflower, poblanos and pepper Jack and spread in the dish. Top with crushed tortilla chips.
Bake for 25 to 30 minutes until bubbling and golden. Rest 10 minutes.