Preheat the oven to 450 degrees . In a bowl, toss the chicken with 1/2 tbsp. oil. Stir in the bell pepper and corn; spread on a large, rimmed baking sheet greased with cooking spray. Roast until the chicken is cooked, about 15 minutes. Place the chicken and the vegetables in a bowl.
Reduce the oven temperature to 350 degrees . In a medium saucepan, whisk 1 tbsp. oil with the flour over medium heat until it bubbles, about 2 minutes. Whisk in the sauce and stock. Increase the heat to high and bring to a boil, whisking, until the sauce thickens, about 1 minute. Stir 1/3 cup sauce into the chicken mixture. Spread 1/2 cup sauce into a 9-by-13-inch baking dish.
Working one at a time, dip tortillas in the remaining sauce, transfer to a plate, fill with the chicken mixture, roll up, and place, seam side down, in the baking dish. Top with the remaining sauce and the cheese. Bake until the sauce bubbles and the cheese melts, 25 minutes. Sprinkle with the cilantro.