In a large pot, heat 2 tablespoons of butter or oil over medium heat. Add 1 diced onion and 2 stalks of diced celery, cooking for about 5 minutes until they soften and become fragrant.

Next, add 8 ounces of thinly sliced mushrooms to the pot. Stir gently and sauté for about 5-7 minutes until they turn golden brown and release their moisture.

Incorporate 3 cloves of minced garlic and 2 teaspoons of fresh thyme into the mushroom mixture. Stir and cook for an additional 1-2 minutes, allowing the garlic to become fragrant.

Pour in 6 cups of chicken stock and 1 cup of heavy cream, stirring to combine all the ingredients thoroughly. Bring the mixture to a gentle simmer and let it cook for about 5 minutes.

Stir in 2 cups of shredded rotisserie chicken and 4 cups of fresh spinach into the pot. Allow the soup to simmer for an additional 5 minutes until the spinach wilts.

Ladle the soup into warm bowls, garnishing with freshly ground black pepper, salt, or chopped parsley if desired. Serve with crusty bread.