Melt the butter: Heat a large Dutch oven over medium heat. Add the butter and let it melt completely.
Brown vegetables: Add the diced onions and sliced mushrooms. Cook for 7 to 8 minutes until the mushrooms are deeply browned and tender.
Add aromatics and flour: Stir in the minced garlic and dried thyme, cooking for 1 minute. Sprinkle the flour evenly over the vegetables and stir constantly for 2 minutes to cook out the raw flour flavor.
Build the broth: Slowly whisk in the chicken stock to prevent lumps. Bring the mixture to a gentle simmer.
Add chicken: Fold in the shredded rotisserie chicken, salt, and pepper. Let the soup simmer gently for 10 minutes.
Finish with cream: Reduce the heat to low. Slowly stir in the heavy cream. Warm through without bringing to a boil, then serve immediately.
