Heat 1 tablespoon oil in a medium saucepan over medium heat. Add flour and chili powder. Cook, stirring, 1 to 2 minutes. Gradually whisk in stock. Add bell pepper, mushrooms, and chicken. Cook, stirring occasionally, until vegetables are tender, 6 to 8 minutes. Remove from heat and stir in cheese.  Heat 1/2 teaspoon oil in a large nonstick skillet over medium heat. Spread half of vegetable mixture on a tortilla, leaving a 1-inch border all around. Top with a second tortilla. Cook, turning once, until golden brown, 2 to 3 minutes per side. Repeat with remaining oil, tortillas, and vegetable mixture.