Heat butter over medium heat until melted.
Add carrots, onions, and mushrooms. Sauté until tender, for about 5 minutes.
Add dried thyme, beef broth, and barley and bring to a boil. Reduce heat and simmer until barley is tender.
While soup is simmering, whisk flour, milk, salt and pepper in a small bowl until smooth.
Stir flour mixture into soup and return to a boil. Stir constantly and cook until thickened.