Put barley and 3 cups water into a soup pot. Bring to boil, skimming off foam. Cook barley until almost tender on medium heat.
In the meantime, melt butter in medium frying pan. Saute onions and garlic until soft.
Once onions and garlic have softened, add mushrooms and ½ teaspoon salt and cook until tender.
When barley is almost tender, add vegetables to soup pot, including liquid expressed as they cook. Add remaining water, tamari, and sherry.
Add several grinds of black pepper.
Simmer 20 minutes, covered, at lowest heat.
Taste to correct seasoning.