Place butter in a large skillet and heat over a moderate flame until the foam subsides. Add the onions and cook slowly, stirring, until translucent, about 8 minutes. Add the mushrooms and cook, stirring, until wilted. Add the wine, garlic, salt, and pepper and simmer until the liquid is almost completely evaporated. Stir in barley and 2 cups of the chicken broth. Simmer slowly, stirring now and then, until the broth is almost absorbed, then add the remaining broth in 1/2-cup incrments. The barley needs about 45 minutes of cooking to become tender. If you are running low on broth while barley is still very underdone, reduce the heat. If you do run out of broth, finish cooking with water. Taste and adjust seasoning.
Tip: Risotto may be made 4 days ahead; cool, then cover and refrigerate. Reheat in a skillet over low flame, tossing frequently.