1. 1. Saute the aromatics: In a large, oven-safe skillet, melt the butter over medium heat. Add the chopped onion, carrots, and celery. Cook, stirring occasionally, until the vegetables are softened, about 5-7 minutes. The onions should become translucent, and the carrots tender-crisp.

2. 2. Create the roux: Sprinkle the flour over the cooked vegetables in the skillet. Stir constantly for 1-2 minutes, allowing the flour to cook out a bit. It should form a thick paste with the butter and vegetables.

3. 3. Whisk in liquids and seasonings: Gradually whisk in the chicken broth, a little at a time, until smooth and no lumps remain. Then, slowly whisk in the milk. Add the dried thyme, black pepper, and salt. Bring the mixture to a gentle simmer, stirring frequently. The sauce will begin to thicken as it heats.

4. 4. Add turkey and peas: Once the sauce has thickened to your desired consistency, stir in the shredded or diced leftover turkey and the frozen peas. Continue to cook for another 2-3 minutes, ensuring the turkey is heated through and the peas are tender. Taste and adjust seasonings if necessary.

5. 5. Top with stuffing: Remove the skillet from the heat. Evenly crumble the leftover stuffing over the top of the turkey mixture, creating a generous crust. You can gently press it down slightly, but do not pack it too tightly.

6. 6. Bake until golden: Place the skillet in a preheated oven at 375 F (190 C). Bake for 15-20 minutes, or until the stuffing topping is golden brown and crispy, and the filling is bubbling around the edges.

7. 7. Serve immediately: Carefully remove the skillet from the oven. Let it rest for a few minutes before garnishing with fresh parsley, if desired, and serving directly from the skillet.