Melt the butter in a large pot over medium heat.
Add the onion, celery, and carrots. Sauté for 5-7 minutes until they are softened.
Add the mushrooms and garlic; cook for another 5 minutes until they are softened.
Stir in the flour and cook for 1 minute to form a light roux.
Slowly whisk in the chicken stock, then add the potatoes, salt, and pepper.
Bring to a boil, then reduce the heat and simmer for 12-15 minutes until the potatoes are tender.
Stir in the shredded chicken, peas, and corn; simmer for 5 more minutes until heated through.
Stir in the heavy cream and parsley; adjust the seasoning to taste and simmer for another 1-2 minutes before serving.