Preheat oven to 450 degrees. In an 8-by-8 baking dish, arrange onions cored side up. Pour broth in bottom of pan. Cover pan tightly with foil. Bake onions until tender, about 20 minutes. Remove from liquid. Drain hot liquid from pan; return onions to pan, cored side up. Set aside.Advertisement
In a medium skillet, cook bacon over medium heat until crispy, 3 to 5 minutes. Add chopped onions; cook, stirring, until golden brown, 10 to 12 minutes or more.
Remove from heat; add bread, cheese, and parsley. Season with salt and pepper; stir to combine.
Fill hollowed onions with chopped-onion mixture (press with back of spoon to pack tightly). Top with more cheese, if desired. Bake, uncovered, until golden brown, 20 to 25 minutes.
To hollow an onion, core onions with a melon baller, leaving a 1/4-inch wall of 1 to 2 layers of onion on sides and on bottom. Reserve insides for chopping. Trim top edge of onion to make a wide, even opening.