Start by heating olive oil or butter in a large pot over medium-high heat. Once the oil is hot, add the diced onion and celery. Stir frequently and sauté for about 5 minutes or until the vegetables start to soften and turn translucent. Adding the celery early on allows its natural sweetness to enhance the flavor base.

Once the onions and celery are softened, toss in the sliced mushrooms. Stir occasionally as the mushrooms release their liquid and begin to brown lightly, which should take around 5 minutes. Adding the garlic and thyme at this stage ensures the flavor builds deeply into the soup.

Pour the chicken stock and heavy cream into the pot. Stir well to combine and bring the mixture to a gentle simmer. Allow the liquid to cook for about 5 minutes, giving the flavors a chance to meld together and the soup to develop a subtle richness.

Once the soup is simmering, stir in the shredded rotisserie chicken and chopped spinach. The warmth of the soup quickly softens the spinach, blending it seamlessly into the broth. Allow everything to cook together for another 5 minutes, ensuring the chicken warms through.

Taste the soup and generously season with salt and freshly cracked black pepper. Squeeze in the juice of one lemon for a bright, tangy finish that elevates the flavors. Once everything is well-blended and seasoned to your liking, the soup is ready to serve.