Cut foil into 12" squares, place chicken in center. Combine oil, lemon juice, thyme and mix well. Spoon over breasts. Seal foil well and place in 350 Dutch oven. Bake 30 min. Open foil and place one slice ham, cheese and tomato over each breast. Bake open for 3 to 5 min. Remove from foil and place on serving platter.