Melt 4 tablespoons of butter in a large saucepan and add the cubed onion, sweat until transparent without coloration. Add the peppers, carrots, and celery and sweat for 1 to 2 minutes on medium heat. Sprinkle the flour on the vegetable/butter mixture and mix well until the flour seems to disappear. Cook for one or two minutes to remove the raw flour taste and add the cold milk a little at a time while whisking energetically. Add salt, pepper, dry herbs, mustard and mix well. Add the thawed corn and the tomatoes at the last minute and set aside.
Cube the turkey meat in medium sized cubes and season with salt and pepper. Melt one tablespoon of butter in a frying pan and sear the meat on medium heat until you get a golden color. Remove the turkey, let it drain on paper towels and add it to the vegetable and the sauce.
Melt the brown bits from the frying pan with some water and add to the sauce. Check the seasoning and set aside.
Butter an oven-proof dish with the remaining butter, add the turkey/vegetable/sauce mixture. Roll the dough out, cut some slits on top, both for decor and to allow the steam to escape when baking. Place the dough on the stew making sure it comes slightly up the sides of the dish.
Place the yolk of the egg in a small dish and add 1 tablespoon cold water, mix and brush the top of the crust. Bake in an oven pre-heated to 400 degrees. Cook for 30 to 40 minutes until the crust is nice and brown and then serve immediately.