Prep the Chicken: If using bone-in chicken, place chicken in a large pot and cover with 8 cups of water. Bring to a boil, then reduce heat and simmer for 45 minutes, or until chicken is cooked through. Remove chicken and set aside to cool slightly. Shred the chicken, discarding the skin and bones. Reserve the broth. If using boneless, skinless chicken thighs, cut into 1-inch pieces.

Sauté Aromatics: In a large pot or Dutch oven, melt butter or heat olive oil over medium heat. Add onion, carrots, and celery and cook until softened, about 5-7 minutes. Add garlic and cook for another minute until fragrant.

Brown the Chicken (If using boneless, skinless chicken): Add the cut chicken pieces to the pot and cook until lightly browned on all sides.

Cook the Mushrooms: Add sliced mushrooms to the pot and cook until softened and browned, about 8-10 minutes. Allow the mushrooms to release their moisture and then reabsorb it for maximum flavor.

Add Broth and Seasonings: Pour in chicken broth (or the reserved broth if using bone-in chicken), add bay leaves, thyme, and sage. Season with salt and pepper to taste.

Simmer: Bring the soup to a simmer, then reduce heat and cook for at least 20 minutes, or up to an hour, to allow the flavors to meld.

Thicken (Optional): If desired, whisk together flour or cornstarch with a small amount of cold water to form a slurry. Slowly whisk the slurry into the simmering soup and cook until thickened, about 2-3 minutes.

Add Chicken (If using bone-in chicken): Stir in the shredded chicken.

Creamy Addition (Optional): Stir in heavy cream or crème fraîche for a richer, creamier soup. Heat through gently; do not boil.

Adjust Seasoning: Taste and adjust seasoning as needed. Add a squeeze of lemon juice for brightness, if desired.

Serve: Garnish with fresh parsley and serve hot.