Preheat the oven to 350°F. Brush the bottom and sides of a 13×9-inch baking dish with butter or nonstick cooking spray. Set aside.
To Make the Filling: In a Dutch oven over medium-high heat olive oil and 2 Tbsp butter. Cook onion and celery for 2 minutes or until they begin to soften and brown. Add cubed potatoes, cooking for 5-7 minutes or until fork tender. Add garlic, cooking for 1 minute longer. To the pot add poultry seasoning, 1 tsp salt, 1 tsp black pepper and tarragon. Mix well. (Using a large pot will double as a mixing bowl.)
To Make the Sauce: In a separate medium saucepan melt reserved 6 Tbsp butter over medium heat. Add flour, 1/2 tsp salt and 1/2 tsp black pepper. Whisk until butter is absorbed, then gradually whisk in chicken stock. Bring to a gentle boil then immediately lower the heat and allow to gently bubble for 4-5 minutes. Whisk in cream and once combined, add shredded cheddar cheese. Stir until completely melted then remove from the heat.
To the Dutch oven add turkey, thawed mixed vegetables, cheese sauce and fresh parsley. Mix well to combine then pour into the prepared baking dish
To Prepare the Pastry: Thaw puff pastry per package instructions, then roll out on a lightly floured surface 2-inches larger than the dish. Transfer to dish, rolling edge to form edges of the crust.
Use a sharp knife to cut 1-2 inch slits in the top to vent. Brush the puff pastry with beaten egg wash, then lightly sprinkle with coarse sea salt.
Bake in the preheated oven for 50-60 minutes until puffed and golden brown.
Allow 5 minutes stand time, then serve.