Sauté the Vegetables: In a large Dutch oven, melt butter over medium heat. Add onion, carrots, and celery. Cook 5–7 minutes, stirring, until softened. Add mushrooms and garlic, cook another 3–4 minutes until fragrant.
Create the Base: Sprinkle flour over the veggies. Stir constantly for 1 minute to form a roux (this prevents a floury taste later).
Add Broth & Potatoes: Slowly whisk in chicken stock. Add potatoes, thyme, salt, and pepper. Bring to a gentle boil, then reduce heat and simmer 12–15 minutes until potatoes are fork-tender.
Cook the Chicken: Stir in chicken pieces. Cook 7–8 minutes until no longer pink inside.
Add Creaminess: Reduce heat to low. Stir in heavy cream until smooth.
Finish the Soup: Add frozen peas, corn, and parsley. Cook 5 minutes until peas are bright green and corn is tender. Taste and adjust seasonings.