Sauté aromatics and vegetables
In a large Dutch oven or heavy soup pot over medium–high heat, melt the 6 Tbsp butter. Add the chopped onion, celery, and carrots. Sauté for 5–7 minutes, stirring occasionally, until the vegetables soften and begin to turn lightly golden.

Add mushrooms and garlic
Stir in the sliced mushrooms and minced garlic. Continue to cook for another 5 minutes or so, until the mushrooms soften and release their moisture.

Incorporate the flour
Sprinkle the ⅓ cup all-purpose flour over the vegetables. Stir continuously for about 1 minute until the mixture becomes lightly golden and you smell the raw flour mellow. This roux will help thicken the soup.

Add broth, potatoes, and seasoning
Gradually pour in the 6 cups of chicken stock, stirring to avoid lumps. Add the Yukon Gold potatoes, 3½ tsp salt (reserve a little for later), and ½ tsp black pepper. Bring the mixture to a boil, then lower the heat to maintain a gentle simmer. Partially cover and cook 12–15 minutes or until the potatoes are just tender.

Add remaining ingredients and finish cooking
Stir in the shredded chicken, frozen peas, and corn. Pour in the ½ cup heavy cream and add the ¼ cup chopped parsley. Bring the soup back to a gentle simmer and cook another 5 minutes, until the peas and corn are warmed through. Taste and adjust seasoning (salt, pepper) as needed.

Serve and garnish
Ladle the soup into bowls and garnish with additional fresh parsley. Optionally, serve with crusty bread or puff pastry bits atop for a pot pie feel.