How to Make It
Preheat oven to 425°F. Arrange onions, cut sides up, in a 9-by-13-inch baking dish. Drizzle with 2 tablespoons oil and season with ½ teaspoon salt and the pepper. Add ½￼ cup water to dish, cover tightly with foil, and roast until onions are tender when pierced with a knife, 45 to 50 minutes. Uncover and let cool slightly.
Meanwhile, combine sausage, parsley, ½ cup Panko, and half the cheese in a medium bowl, breaking up sausage as you mix.
When onions are done, remove inner layers of each onion half, leaving outer few layers intact, and transfer to a food processor; pulse until roughly chopped. Add chopped onions to sausage mixture and mix until evenly combined. Divide sausage mixture among onion halves.
Combine remaining cheese, ½ cup Panko, 1 tablespoon oil, and ￼¼ teaspoon salt in a medium bowl and mix with your fingers until evenly combined. Dividing evenly, place panko mixture on top of sausage mixture. Return dish to oven and bake, uncovered, until tops are golden, 20 to 25 minutes. Serve topped with parsley leaves.