Heat olive oil or butter in a large pot over medium heat. Add the diced onion and finely chopped celery, cooking until they become soft and fragrant.
Once the aromatics are ready, add the thinly sliced mushrooms to the pot. Cook them until golden brown, which will enhance their flavor significantly.
Stir in the fresh thyme and crushed garlic, allowing them to cook until fragrant. This step infuses your soup with rich herbal notes.
Pour in the stock along with the cream, stirring gently as you bring everything to a simmer for about five minutes. This creates a luscious broth that forms the backbone of your soup.
Add in your shredded rotisserie chicken followed by the chopped spinach, letting everything simmer together for another five minutes until heated through.
Finally, season your hearty soup with salt, pepper, and a splash of lemon juice to brighten up all those wonderful flavors before serving hot!