Cook frozen vegetables according to package directions. Drain and set aside. In a large saucepan, saute onion and mushrooms in butter until tender. Stir in the flour, salt, sage and pepper until blended. Combine the water, milk and bouillon. Slowly add to saucepan, stirring constantly.
Bring to a boil. Cook and stir for 2 minutes or until thickened and bubbly. Stir in the chicken, pimientos, parsley and reserved peas and carrots. Spoon into six individual baking dishes.
Roll and cut pastry into circles 1 in. smaller than top of casseroles; place one circle on each. Bake at 425° for 12-15 minutes or until crust is lightly browned.