Heat a large heavy-bottomed pot over medium heat, and add a splash of oil. Once shimmering, add diced onions and sauté until translucent and fragrant, about 5 minutes.
Add sliced mushrooms to the pot, stirring occasionally. Cook until they release their juices and turn a rich golden brown, about 8 minutes.
Push the mushrooms to the sides of the pot, then add minced garlic to the center. Cook for about 30 seconds until fragrant, stirring constantly to prevent burning.
Pour in the chicken stock, scraping up any browned bits from the bottom. Bring the mixture to a gentle simmer and cook for 10 minutes to develop flavors.
Add shredded cooked chicken to the pot, stirring to combine, then pour in the heavy cream. Continue simmering for another 5 minutes until the soup is heated through and creamy.
Taste the soup and season with salt and pepper as needed. Stir in lemon juice for brightness and sprinkle chopped herbs for a fresh herbal lift.
Serve hot, ladling the creamy mushroom chicken soup into bowls. Enjoy its velvety texture and earthy flavors in every spoonful!