Cut margarine into flour in a medium bowl until it resembles coarse meal. Mix in just enough club soda to form a ball. Refrigerate dough for 3 hours.
Preheat the oven to 400 degrees F (200 degrees C). Assemble 4 oven-safe onion soup bowls.
Combine mixed vegetables, water, and leftover chicken in a medium saucepan; bring to a boil. Add cornstarch and chicken bouillon; cook and stir until mixture has thickened, 3 to 5 minutes. Set aside.
Remove dough from the refrigerator and divide into 5 balls. Roll 4 of the balls on a lightly floured surface into circles that are large enough to cover the bottom and sides of the soup bowls. Roll out the last ball of dough. Cut out 4 circles, each slightly larger than the size of each bowl opening.
Press bottom crusts into each soup bowl and fill with about 1 1/2 cups filling on top. Top with smaller dough circles and press dough together to seal.
Bake in the preheated oven until crust is golden brown, 45 to 50 minutes. Let stand 10 to 15 minutes before serving.
You can use turkey in place of chicken.