In a large skillet sauté onion, celery, carrots, and mushrooms for about 5 minutes until they are tender.
Add the ground turkey and sauté until cooked through. Season with salt and pepper, then remove from pan and set aside.
In the same pan, add flour and spices and sauté for approximately 2 minutes, stirring constantly.
Add in the tomato paste, and stir for another minute, stirring constantly.
Add the broth, Worcestshire sauce, and Italian seasoning and simmer until it thickens.
Add the peas and meat mixture back into the pan and simmer for an additional five minutes.
Place the filling in a 9x13 glass casserole dish and set in the fridge to cool.
While it cools, place 4 cups of diced potatoes into a pot of water and boil for 20 minutes or until potatoes or tender.
Remove all but about 1 cup of the potato water, and then add 1⁄2 a cup of butter and a splash of milk. Once butter has melted mash up the potatoes until they are smooth and creamy and add salt and pepper to taste.
Once the filling has cooled, top with the mashed potatoes.
Place the Shepherds pie into the oven and bake at 400 for about 20 minutes or until the filling is bubbling. Broil for an additional 5 minutes or so if a crispy top is desired.