Easy Cream Of Mushroom Soup

Sauté: Melt butter in a large heavy-bottomed soup pot over medium-high heat. Add garlic and onion and cook, stirring until starting to brown and soften, 4 to 6 minutes.
Cook Mushrooms and Season: Add mushrooms salt, thyme and pepper, and cook stirring often until the mushrooms have released their juices and the juices have evaporated, 5 to 7 minutes. Add sherry and rosemary, bring to a simmer and let sherry evaporate until dry.
Add Flour To Thicken: Sprinkle flour over the vegetable mixture and stir to coat.
Add Liquid: stir in broth, increase heat to high and bring to a simmer, stirring continuously until the soup has thickened slightly. Add milk and return to a simmer, stirring often.
Finish and Serve: Remove from the heat, stir in chives. Ladle into bowls and drizzle with truffle oil, if using.