Preheat the oven to 400°F.

In a large oven-safe skillet, melt the butter over medium heat. Add the diced onions, carrots and celery, then sauté until the vegetables begin to soften, about 5 minutes. Stir in the garlic and cook for an another minute until fragrant.

Sprinkle the flour over the sautéed vegetables and stir to create a roux. Cook for 2-3 minutes, allowing the roux to turn slightly golden in color.

Deglaze with white wine, sherry and chicken stock, stirring constantly until the mixture thickens and becomes smooth. Allow it to simmer for a few minutes, then turn off the heat. 

Add the heavy cream, thyme, peas, the shredded leftover turkey, season wit salt and pepper, then stir to combine. Taste and adjust seasoning as needed. 

Roll out the thawed puff pastry sheet on a lightly floured surface so it's just large enough to fit the skillet with a slight overhang.

Place the rolled-out puff pastry on top of the pot pie filling in the skillet, tucking in the excess around the edges to form a crust.  

If desired, brush the puff pastry all over with the beaten egg to give it a beautiful golden-brown finish when baked. You can also sprinkle some flaky sea salt, fresh herbs or black pepper on top. 

Cut a few slits in the top to let steam escape, then bake for about 30 minutes or until the pastry is puffed and golden, and the filling is bubbling around the edges.

Let cool slightly before serving.