Preheat the broiler. Toss the cauliflower, poblano and scallions with the vegetable oil, chili powder, cumin and 1/2 teaspoon salt on a rimmed baking sheet; spread in a single layer. Broil until the vegetables are browned around the edges, 7 to 10 minutes.
Meanwhile, make the biscuits: Whisk the cornmeal, flour, baking powder, brown sugar and 3/4 teaspoon salt in a medium bowl. Work the butter into the cornmeal mixture with your fingers until crumbly. Stir in the buttermilk and 1/3 cup cheese with a fork until combined.
Remove the baking sheet from the broiler and preheat the oven to 475 degrees F. Add the beans and their liquid, the tomato sauce, corn, vegetable broth and 1/2 teaspoon salt to the roasted vegetables and gently stir. Spoon the biscuit dough on top in 12 mounds, about 1 1/2 inches apart. Sprinkle the biscuits with the remaining 1/3 cup cheese.
Bake until the chili starts bubbling and the biscuits are golden and cooked through, 25 to 30 minutes. Sprinkle with scallions.