Make the Soup Base:
Heat a splash of oil or a knob of butter in a large pot or Dutch oven over medium-high heat. Add the diced onion and chopped celery, and cook for about 5 minutes until soft and translucent. Stir in the mushrooms and cook until golden brown, about 5 more minutes. Add the thyme and minced garlic, and cook until fragrant, about 1 minute.
Cook the Soup:
Pour in the chicken stock and heavy cream, then bring the mixture to a simmer. Let it cook for about 5 minutes to allow the flavors to meld.
Add Chicken and Spinach:
Stir in the shredded rotisserie chicken and spinach. Continue to simmer for another 5 minutes, or until the chicken is heated through and the vegetables are tender.
Season and Serve:
Season the soup with salt and black pepper to taste. Add lemon juice to brighten the flavors. Serve the soup hot with your favorite bread for dipping.
