Prepare the pastry shells according to the package directions.
Heat the oil in a 10-inch skillet over medium-high heat. Add the onion and cook until tender.
Stir the soup, milk and peas and carrots in the skillet and heat to a boil. Reduce the heat to low. Cover and cook for 5 minutes or until the vegetables are tender. Add the chicken and cook until the mixture is hot and bubbling. Spoon the chicken mixture into the pastry shells.