In a Dutch oven, bring a small amount of water to a boil. Peel onions; using a slotted spoon, place onions in boiling water. Cook for 4-6 minutes more until softened; remove and let stand until cool enough to handle. Cut a 1/4-in. slice off the top of each onion. (Slice a little more off if having trouble removing the insides.) Remove center, leaving a 1/2-in shell. Chop centers and tops of onions; set aside. In a skillet, cook bacon until crisp; remove to paper towel to drain. In the drippings, saute chopped onion until tender. Stir in bread crumbs and parsley. Add the butter and mushrooms; cook until mushrooms are tender. Add bacon, salt, pepper and nutmeg. Stuff onion shells; place in an ungreased shallow 1-qt. baking dish. Pour broth around onions. Bake, uncovered, at 375° for 45 minutes or until tender, basting outside of onions frequently during the first 15 minutes. Sprinkle with parsley if desired.
Other nice additions: minced garlic, chopped red and/or green bell pepper.