Preheat oven to 350 degree F. Grease a 2-quart casserole dish with butter.
Melt butter in a large sauté pan over medium to medium-low heat. Add onion and sauté until tender and translucent, but not brown. Season with salt and pepper.
Add flour to onions and stir 1-2 minutes. Slowly whisk in stock starting with 2 1/2 cups and adding more at the end if casserole mixture is too thick. Adjust heat to a simmer and cook until roux thickens. Whisk in cream.
Stir in turkey, vegetables, potatoes and parsley. Place in prepared casserole. Top with crumbled leftover stuffing. Bake for 30 minutes or until casserole bubbles and is hot.