In a large frypan saute the onions in butter over medium heat until just light golden brown (about 8 minutes).
Add in mushrooms and garlic and saute for another 5-7 minutes more.
Add in barley; saute until lightly browned (this should take about 7-8 minutes).
Add in 2 cups broth and pepper to taste (no need to add salt as the stock has added salt) bring to a boil, cover and reduce heat.
Transfer to a large greased casserole dish.
Bake for about 45 minutes.
Remove from oven,add in remaining 1 cup broth.
Return to oven and bake for 30 minutes more.
Add more liquid if the mixture becomes too dry.
Garnish with chopped parsley if desired Note: grated Parmesan cheese or grated cheddar cheese may be mixed in to the warm casserole after baking if desired, also the recipe may be prepared on top of the stove in a large saute pan, instead of baking in the oven if desired.