Melt the 6 Tbsp butter in a Dutch oven or soup pot over medium/high heat, then add the onion, celery, and carrots and cook for 5 to 7 minutes, stirring now and then. The vegetables should soften, pick up a little color, and smell sweet and buttery.

Stir in the mushrooms and garlic, then cook for 5 minutes more until the mushrooms release moisture and turn tender. The pot should smell deeper and more savory, with the garlic fragrant but not browned.

Sprinkle in the 1/3 cup flour and stir constantly for 1 minute to coat the vegetables and butter evenly. The mixture should look like wet sand and smell lightly nutty, with no dry flour pockets.

Pour in the 6 cups chicken stock, then add the potatoes, 3 1/2 tsp salt (or to taste), and 1/2 tsp black pepper; bring it to a boil, lower to a simmer, and cook partially covered for 12 to 15 minutes. The broth should bubble gently, and the potatoes should be just tender when pierced, not falling apart.

Add the shredded chicken, peas, corn, whipping cream, and parsley, then return the soup to a gentle simmer for 5 minutes and season to taste before taking it off the heat. The soup should look creamy and thick, and the peas and corn should be tender and bright.