Preheat oven to 350°F. Mix tomato sauce, stock, Seasoning and flour in small saucepan until well blended. Cook on medium heat until bubbly and thickened, about 5 minutes, stirring occasionally. Set aside.
Heat oil in medium skillet on medium-high heat. Add onion and mushrooms; cook and stir 4 to 5 minutes or until softened. Stir in chicken and 1/4 cup of the sauce; cook until heated through.
Spoon about 1/4 cup each chicken filling and cheese onto each tortilla. Roll tortillas around filling. Place seam-side down in 13x9-inch baking dish sprayed with no stick cooking spray. Pour remaining sauce over enchiladas. Sprinkle with remaining 1 cup cheese.
Bake 20 minutes or until heated through and cheese is melted.