Don't rush the cooking of the chopped onions. Keep them on the stove until the natural sugars are caramelized, yielding a deep, rich, sweet flavor.
- 4 medium (8 to 10 ounces each) yellow onions, peeled and hollowed; insides chopped
- 1/2 cup reduced-sodium chicken broth
- 4 slices bacon, cut crosswise into 1/2-inch pieces
- 2 slices white sandwich bread, torn into small pieces
- 4 ounces Gruyere (or Swiss) cheese, shredded (1 cup), plus more for garnish (optional)
- 2 tablespoons chopped flat-leaf parsley
- Coarse salt and ground pepper
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.
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.