Saute onions and garlic in olive oil. Add salt and pepper. Add sliced yellow squash to pan. Cook slightly. Add petit diced tomatoes. Heat a few minutes. Right before serving, add cream. Heat 1 - 2 minutes (do not boil).
Cut potatoes lengthwise into slices. Place on 18 X 12 in. double layer of foil. Sprinkle with salt. Add peppers and tomatoes. Lay chicken on top of veggies and brush with oil. Mix ketchup, oregano and butter. Pour over chicken. Wrap foil securely around chicken and veggies. Bake for 1 hour at 400 degrees, or grill 5 inches from coals for 25 - 30 minutes each side.