Roasted Chicken White Chili

Name: Roasted Chicken White Chili
Ingredients: 1 pound boneless, skinless chicken thighs; 1/4 cup vegetable oil, plus more for drizzling; Kosher salt and freshly ground white pepper; 3 celery stalks, cut into medium dice (about 1 cup); 1 medium onion, cut into medium dice (about 2 cups); 1 yellow bell pepper, cut into medium dice (about 1 cup); 1/4 cup all-purpose flour; 4 cups (1 quart) chicken stock, warmed; Two 15-ounce cans Great Northern beans; Two 4-ounce cans chopped green chiles; 2 teaspoons garlic powder; 2 teaspoons ground cumin; 1 teaspoon minced fresh rosemary; 2 green tomatoes, cut into large dice (about 2 cups); 1/2 to 1 teaspoon cayenne, depending on how much spice you like; Sour cream, for garnish