Cheesy Scallion Stuffing with Sesame Seeds

Name: Cheesy Scallion Stuffing with Sesame Seeds
Ingredients: 1 (1-pound) sourdough bread loaf; ¼ cup unsalted butter, plus softened unsalted butter, for greasing; 1 large (12-ounce) red onion, halved lengthwise and thinly sliced (about 21/2 cups); 7 large scallions (about 7 ounces), thinly sliced diagonally (about 2 cups); 2 medium celery stalks (about 2 ounces), thinly sliced (about 1 cup); 1 tablespoon granulated sugar; 1 teaspoon kosher salt; 1 ½ cups turkey stock or vegetable stock; 6 ounces Parmesan cheese, shredded (about 1 1/2 cups), divided; ½ cup whole milk; 2 tablespoons plus 1 teaspoon toasted sesame seeds, divided; 1 teaspoon soy sauce; 1 teaspoon toasted sesame oil; 2 large eggs, lightly beaten