Put barley into a sieve, wash thoroughly under cold, running water to release the starch. Then place in bowl of cold water and leave the barley to soak for about 4 hours. Drain.
In pan, place 1 tablespoon butter with 1-1/4 cups water and salt; bring to a boil. Add drained barley, cover the pan and simmer very gently on low heat for about 30 minutes, or until the barley is soft but still holds its shape. It should just have absorbed the water by then.
Preheat oven to 325 degrees. Melt remaining butter in a skillet. Saute onion and garlic until onion is limp and translucent. Add mushrooms, shake the pan and cook for 3 to 4 minutes. Season with salt and pepper to taste. Add the broth, cover the pan and simmer for 10 minutes. Stir in half the chopped parsley.
Spread half the barley to cover the bottom of a greased 5-cup ovenproof dish. Cover with the mushroom mixture and spread the remaining barley over the top.
Mix together the egg, yogurt, and cheese and season with salt and pepper. Pour the topping carefully over the barley.
Bake at 350 degrees for 20 minutes, until the custard topping has set. If you like a well-browned top, put the cooked dish under a hot-broiler for 2 to 3 minutes. Sprinkle with remaining parsley for garnish. Yield: 6 servings.
Note: "Use a cheese not too heavy in taste. I use Parmesan lite."