1. In a large pot over medium heat, melt the butter then add the onion. Cook, stirring often, until the onion softens and becomes translucent, 3-5 mins. Add the mushrooms, salt, pepper, and nutmeg. Cook, stirring, until the mushrooms are tender, 7-10 mins. (Option to remove a few mushroom slices to serve on top of the soup at the end.)
2. Mix in the garlic and flour. Cook for 1 min, then add the chicken stock 1 cup at a time, stirring to incorporate before adding more. Bring to a simmer, then stir in the milk 1 cup at a time. Bring back to a simmer and cook for 3-5 mins until the broth thickens slightly.
3. Remove from the heat and transfer about three quarters of the soup to a blender (or use an immersion blender). Blend until smooth, then return the soup to the pot. Stir in the parsley and balsamic vinegar, taste, and add more salt or pepper if desired.
4. Serve warm with a reserved mushroom slice on each bowl (if using), a sprig of parsley, and a drizzle of high quality olive oil. Enjoy!
