Boil 1 cup broth to a boil in a large pot. Add chopped onion and cook 3 minutes until soft, then add remaining cup broth, potatoes and thyme. Bring to a boil and cook until potatoes are tender. Add mixed frozen veggies and mushrooms. Meanwhile, combine the cornstarch and milk in a small bowl, stir until smooth, add mustard. Add to veggies with salt and pepper to taste. Cook for 5 minutes or until thickened. Stir in cooked chicken and remove from heat. Make dough from remaining ingredients. Combine flour and salt. In a separate bowl, thoroughly mix oil and milk. Create a well, pour milk mixture into the flour. Form into a ball. Roll out to cover 9x13 pan.