Heat the butter and olive oil in a medium dutch oven over medium heat. Add the shallots and mushrooms along with the thyme. Cook, stirring occasionally, for 5-7 minutes. Add the minced garlic and cook, stirring constantly, for 1 minute. Add the chicken broth and bring to a boil. Reduce heat to low. Remove half of the mushrooms from the soup; set aside. Blend the soup with an blender or immersion blender until creamy. Add the mushroom slices back into the soup along with the milk. Simmer for 10 minutes. Season with freshly cracked pepper and truffle salt (if you have it) or sea salt, to taste. Serve with additional thyme leaves on top if desired. Enjoy.