Measure out 4 cups of turkey broth or leftover turkey gravy. If using gravy instead of stock, it needs to be quite thin. If the gravy was on the thick side even when it was hot last night at dinner, thin it down with a little chicken stock. If you do not have enough turkey broth or leftover turkey gravy, measure what you have and add some chicken broth until you have approximately 4 total cups of liquid. When I made it last time, I had to add nearly 1 cup of chicken broth because I only had 3 cups of leftover turkey gravy. It tasted just fine.
In a 10" skillet with straight sides, melt the butter over medium heat. Add the scallions, peas, and carrots and cook for 2 minutes. Add the garlic and cook 1 minute longer. Add the turkey then the turkey broth/gravy/chicken broth and stir to combine. Bring to a simmer and taste for seasoning. Add salt and pepper to taste. Don't under season the broth. Once the dumplings are added, the flavors will be muted so season the broth a little more robustly than you normally would.
While you are waiting for the broth to come to a simmer, make the dumplings by whisking together the flour, baking powder, and salt. When the broth mixture comes to a simmer and you have adjusted the seasonings, add the milk and melted butter to the flour mixture and use a spoon to stir everything together until it is just combined. Don't over mix or you'll toughen the dumplings.
I use two soup spoons to add the dumpling dough to the broth. I scoop up some of the dough with one spoon and push it off into the broth with the other. Continue adding dough as quickly as you can – leaving a little room between dumplings, if possible. It'll be a tight squeeze to get all of the dough into the broth but it'll fit. IMMEDIATELY cover the dumplings with a tight fitting lid and cook for 20 minutes. Whatever you do, DON'T PEEK while they're cooking. The dumplings should lightly simmer while cooking. I normally reduce the heat to someplace between medium low and medium after the first 5 minutes of cooking so the liquid does not simmer too aggressively.
After 20 minutes, take the skillet off the heat and serve piping hot. This is an EXCELLENT way to use up Thanksgiving leftovers. Delicious and easy!
TIP: This recipe has a high ratio of dumplings to broth – which means that it is not particularly soupy when fully cooked. If you like a lot of soup with your dumplings, use the same amount of gravy base but halve the dumpling recipe.