1. Pierce sweet potato several times with a fork. Place in microwave oven, and cover with a damp paper towel. Microwave at HIGH 3 minutes or until done. Let stand 5 minutes; peel and dice. Set aside.
2. Sprinkle chicken evenly with seasoned salt. Sauté chicken and onion in hot oil in a Dutch oven over medium-high heat 5 to 8 minutes or until done. Remove chicken and onion.
3. Melt butter in Dutch oven over medium-high heat; whisk in flour, chicken broth, and soup. Reduce heat to medium-low, and cook, stirring occasionally, 3 to 4 minutes or until thickened. Stir in cooked chicken and onion, sweet potato, peas and carrots, lemon juice, and pepper. Cook, stirring often, 5 minutes or until thoroughly heated. Spoon chicken mixture into a lightly greased 13- x 9-inch baking dish.
4. Roll piecrust into 13- x 9-inch rectangle; fit over chicken mixture in baking dish. Cut several slits in top of crust for steam to escape.
5. Bake at 400° for 30 to 35 minutes or until crust is golden brown and filling is thoroughly heated. Let stand 10 minutes before serving.