Put oil in the bottom of a large frying pan and turn heat to medium low. Add onions and sprinkle a bit of salt to draw the liquid out. Sautée until translucent (a few minutes). If using Rosemary, remove leaves and mince. Add to onions. If using Sage, chiffonade and add.

While the onions are cooking, place potatoes/sweet potatoes into a medium sized pot. Cover with water and add about a teaspoon of salt. Bring to a boil and reduce to simmer. Cook for about 6 minutes. Test the potatoes with a fork. You should be able to pierce with a fork without the potatoes falling apart.

While the potatoes are cooking, add the celery and carrots to the onions. Stir so the carrots, celery and onions are covered in oil and herbs. Continue to stir occasionally so they don't stick to the bottom of the pan. If using mushrooms, add them now.

Remove the potatoes from the stove and drain them. Add them to the onions, celery and carrots. Add the turkey and ¾ cup of stock. Simmer for 5-10 minutes. Taste. Add salt, pepper and more herbs to your liking.

Mix the remaining ¼ cup of stock with 1 tablespoon of corn starch. Make sure there are no lumps. Pour the cornstarch slurry over your pot pie filling. Stir and pour at the same time to mix the cornstarch throughout and prevent lumping. If it is too thick, add a bit more stock or even water. If it is too thin, remove some of the liquid from the pan and add ½ tablespoon of corn starch and repeat.

Once it is the thickness and flavour that you like, remove from heat.. If you are including peas, add them now. Pour your pot pie mix into your baking dish.

Preheat oven to 375 degrees.

Unroll puff pastry and lay over pot pie filling. If you have extra, you can crimp the edges. You can use a pairing knife to make some slits in the top to vent steam – it also looks pretty.

Beat one egg and add 1 tablespoon of milk or water. Brush mixture on top of puff pastry.

Bake for 15-20 minutes until golden brown. Allow to stand for 10 minutes and serve.