Preheat oven to 400˚F.

In a deep skillet, heat the olive oil over medium heat.

Season the chicken breasts on both sides with salt and pepper. Add to the skillet.

Cook the chicken until golden brown and flip to continue cooking on the opposite side. Cook until the chicken is no longer pink, approximately 6-7 minutes on each side.

Transfer to a plate and allow to rest.

While the chicken cools, whisk together the cornstarch and half and half. Set aside.

Add the onion, celery and carrot to the skillet. Cook over medium heat until the vegetables begin to soften, approximately 3-4 minutes.

Push the onion mixture to the sides of the pan and add the mushrooms, if using. Cook until the mushrooms begin to brown and then flip them over to continue cooking for an additional 1-2 minutes.

Stir in the garlic and cook for 1 minute or until fragrant.

Add the peas and stir all the vegetables together.

Stir in the Italian seasoning, garlic powder, onion powder, nutmeg, cayenne and thyme. Cook for 1 minute.

Whisk in the chicken stock and bring to a boil. Turn the heat to low and stir in the chicken.

Add the half and half and continue to simmer until slightly thickened.

Stir in the salt and pepper. Transfer the mixture to a 9x13 baking dish coated with cooking spray.

Cover the chicken pot pie with the puff pastry and cut a few slits in the top to allow the steam to escape.

Brush the top of the pastry with the egg and sprinkle with the parmesan cheese.

Transfer to the oven. Bake for 15-20 minutes or until the pastry is puffed up and golden brown.

Remove from the oven and allow to cool for 5-10 minutes before serving.

Serve hot topped with fresh chopped thyme, if desired.