In a large stock pot over medium heat, melt 3 tablespoons (42g) of unsalted butter and add the chopped celery, carrots, onions, potatoes, and garlic. Sauté the vegetables until softened but still with a little bite (i.e., not mushy). This should take 10-15 minutes.
Transfer the sautéed vegetables to a bowl and set aside.
Melt the remaining butter in the pot, sprinkle the flour over the butter, and use a wooden spoon to stir the mixture for about 1 minute. This will cook off the raw flavor of the flour.
Slowly whisk in the chicken stock and milk to avoid making lumps. Add the salt, pepper, parsley, thyme, and bay leaf and continue to whisk the sauce on medium-low heat until the sauce is very thick.
Add the cooked turkey, peas, corn, and sautéed vegetables. Stir to combine and heat through. The filling can be refrigerated for 1-2 days at this point.