Heat a large heavy-bottomed pot over medium heat and add a tablespoon of oil. Once shimmering, add the diced onion and cook, stirring often, until translucent and fragrant, about 5 minutes.
Add the sliced mushrooms to the pot and cook, stirring occasionally, until they release their juices and turn golden brown, about 8 minutes.
Push the mushrooms to the sides of the pot and add the minced garlic to the center. Cook for about 30 seconds until fragrant, stirring constantly to prevent burning.
Pour in the chicken stock and bring the mixture to a gentle simmer. Let it bubble softly for 10 minutes to develop flavor.
Stir in the shredded cooked chicken and add the heavy cream, mixing well. Continue to simmer for another 5 minutes until the soup is heated through and slightly thickened.
Taste the soup and season with salt and pepper as needed. If it feels too thick, add a splash more chicken stock to thin it out.
Finish by stirring in fresh lemon juice and sprinkling chopped herbs over the top. Give it a final stir and serve hot with crusty bread if desired.
