1. Preheat oven to 400 degrees. Slice off tops and bottoms of onions, leaving at least a 2-inch diameter exposed at the top. Scoop out the inside of each onion (about halfway down) using a melon baller or a spoon. Season insides with salt. Transfer onions to a baking dish, and cover with parchment, then foil. Bake until just starting to soften, about 1 hour.

2. Meanwhile, melt butter in a large skillet over medium heat. Crumble sausage into skillet, and cook, stirring, until almost cooked through, about 3 minutes. Add apple and fennel seeds, and cook until sausage is no longer pink, about 2 minutes.

3. Drain sausage mixture, reserving juices in a medium bowl. Finely chop sausage mixture, and add to bowl. Stir in breadcrumbs, parsley, sage, and 1/4 cup Gruyere. Let cool.

4. Fill onions with stuffing (about 3 tablespoons each), then top with remaining 1/2 cup Gruyere. Bake until tops are crisp and brown, about 20 minutes more.