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 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 vegan cheese with a fork until combined.
Remove the baking sheet from the broiler and preheat the oven to 475°. 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.