OPTIONAL: you can add 1 cup each of leftover green beanschopped and/or 1 cup leftover roasted root vegetable pieces
1/2cupall-purpose flouror more, as needed
1 1/3cupmashed potatocool or room temperature
1large egglightly beaten
Heat butter in a large pot over medium heat. Add onion, celery, and carrot; cook until soft. Stir in flour to coat vegetables, then slowly stir in chicken stock and milk. Add turkey, bay leaf, thyme, salt, pepper (and optionally any leftover veggies you'd like to include) and bring to a simmer.
In a large bowl, whisk together flour and salt. Add mashed potato and beaten egg, using a fork to incorporate. Knead gently in the bowl until a dough comes together. It will be sticky, but if wet to the point of falling apart (which may happen if you added milk or cream to your mashed potatoes), add more flour, a tablespoon at a time, until a soft, scoopable dough forms.
Drop dough by golfball-sized pieces into the simmering stew. Cover and cook 15 minutes, until dumplings have nearly doubled in size and are springy to touch.