In a large pot or Dutch oven, heat a little oil or butter over medium-high heat.
Add the chopped onion and celery, and sauté for about 5 minutes until they soften and become translucent.
Next, add the mushrooms to the pot and cook for an additional 5 minutes, allowing them to release their moisture and brown.
Stir in the fresh thyme (or dried thyme) and minced garlic, and sauté for another minute until the garlic becomes fragrant.
Pour in the chicken broth and heavy cream, stirring everything together.
Bring the mixture to a gentle simmer and cook for about 5 minutes, letting the flavors combine.
Stir in the shredded rotisserie chicken and spinach.
Continue to simmer the soup for another 5 minutes, or until the chicken is heated through and the spinach has wilted.
Taste the soup and season with salt and black pepper according to your preference.
Stir in 1-2 tablespoons of lemon juice to balance the richness and add brightness to the flavor.
Serve the soup hot with your favorite crusty bread on the side for dipping.
