Start by heating a splash of olive oil in a large pot over medium heat. Add the diced onion, sliced carrots, chopped celery, and minced garlic, sautéing until softened, which should take about 5-7 minutes.
Next, stir in the sliced mushrooms and allow them to cook until they release their juices, which usually takes around 3-4 minutes.
Pour in the chicken broth and bring everything to a gentle simmer.
Add your shredded rotisserie chicken along with the dried thyme into the pot. Let it simmer for about 20 minutes.
Finally, taste your soup and adjust seasoning with salt and pepper as needed. Ladle it into bowls while it's hot and enjoy!