Melt the butter in a large pot over medium-high heat. Add the carrots, celery, onion and a pinch of salt and cook, stirring, until soft, 12 to 15 minutes. Stir in the flour and cook for another minute. Stir in half of the milk and cook, stirring, until thickened, about 4 minutes. Stir in the remaining milk and cook, stirring, until very thick, about another 3 minutes. Add the bouillon powder, rosemary, a bunch of turns of black pepper and salt to taste.
Layer a third of the soup on the bottom of a 9-by-13-inch casserole dish, then layer on half the wild rice and half the turkey. Repeat, then top with the remaining third of the soup mixture. Top with the stuffing and bake, covering the top with foil if it starts to get too brown, until bubbly, about 45 minutes.
Serve with leftover cranberry sauce.
Since it's impossible to predict exact leftover amounts, don't fret if you have a little more or a little less turkey or stuffing! These amounts are just a ballpark.