Wash mushrooms, trim stems and slice. Set aside.
Melt butter in large nonstick soup pot or large deep nonstick pan over medium heat. Do not let butter brown. Add onions, leeks, carrots, celery, and garlic. Sauté until tender, about 6 minutes, stirring quite often. Add mushrooms. Cook 3 minutes.
Pour in chicken broth, barley, potato, and bay leaves. Bring to boil over high heat, reduce to low, cover and simmer about 50 minutes. Season to taste with fresh ground pepper.
Season to taste with fresh ground pepper. Remove bay leaves and discard. Cool slightly.
Store cooled soup in the fridge overnight. This soup is best served the next day. The flavors come alive.