6 medium yellow onions, trimmed, peeled, and halved pole to pole 3 tablespoons olive oil, divided ¾ teaspoon kosher salt, divided ¼ teaspoon freshly ground black pepper 12 ounces frozen sweet Italian sausage (about 3 links), defrosted, casings removed ½ cup chopped parsley leaves, plus more for serving 1 cup Panko, divided 4 ounces grated white Cheddar (about 1 cup), divided