Heat a large heavy-bottomed pot over medium heat and add oil until shimmering.
Add sliced mushrooms to the pot and cook, stirring occasionally, until they turn golden brown and release their aroma, about 8 minutes.
Pushing the mushrooms to the side, add diced chicken to the empty space and cook until browned on all sides, about 5-6 minutes, stirring occasionally.
Pour in chicken broth and use a wooden spoon to scrape up any browned bits from the bottom of the pot, then bring to a gentle simmer.
Stir in minced garlic, thyme, and heavy cream, then season with salt and pepper to taste. Bring the mixture to a gentle simmer and cook for 10 minutes, allowing flavors to meld and the soup to thicken slightly.
Finish by squeezing in lemon juice for a fresh brightness, then give everything a gentle stir.
Taste and adjust seasoning with more salt, pepper, or lemon juice if desired.
Serve the soup hot, ladled into bowls, garnished with a sprig of thyme or a drizzle of cream if you like.