1. Preheat the oven to 400 F. Spray a 9 x 13” pan with non-stick spray and set aside.
2. Prepare the stuffing as per the directions on the box, or measure out 3 cups of leftover stuffing and set aside.
3. In a large saucepan over medium heat, melt the butter. Add the diced onion and sauté for 4-5 minutes or until the onion turns translucent. Add the minced garlic and sauté for another minute, before adding in the flour, salt and pepper and stirring to combine, about 1 minute.
4. Slowly pour in the stock, stirring constantly. Add the heavy cream, and bring the sauce to a simmer. While continuing to stir, simmer the sauce until it has thickened.
5. Remove the mixture from heat, then add in the mixed vegetables, followed by the diced turkey. Stir to combine, then pour everything into the greased casserole dish
6. Prepare the topping. Scatter the shredded cheese over the top of the dish. In a small dish, stir together the Panko crumbs with the melted butter. Scatter the crumbs all over the top of the shredded cheese.
7. Bake the casserole for 25-30 minutes or until lightly browned on top and the casserole is bubbling. Remove from the oven and let the casserole cool for 5-10 minutes before serving.

Leftovers, once completely cooled, can be stored in an airtight container in the fridge, for 2-3 days.