Preheat the oven to 450 degrees . Grease a baking sheet with 2 tablespoons olive oil. In a small bowl, combine the chili powder, 1 teaspoon salt and 1/2 teaspoon pepper. Place the tomatoes, seed side up, and onion on the prepared baking sheet. Stud each tomato with a garlic clove half. Drizzle the remaining 1/4 cup olive oil over the vegetables and dust with some of the chili powder mixture. Bake until the onions begin to brown, 25 to 30 minutes.
Meanwhile, roast the chiles directly over a stovetop gas flame, turning occasionally with tongs, until charred all over. Transfer to a bowl, cover tightly and let cool slightly. Peel, stem and discard all the seeds. Chop the chiles.
In a large saucepan, combine the roasted tomatoes, 2 roasted onion slices and the roasted garlic. Add the vegetable broth. Using an immersion blender, puree the mixture. Chop the remaining roasted onion and add to the saucepan. Stir in the chiles, black-eyed peas, cumin and remaining chili powder mixture. Bring to a boil, lower the heat and simmer until thickened, about 15 minutes. Stir in two-thirds of the cheese to melt, then top with the remaining cheese.