sausage stuffing recipe

8 cups store-bought unseasoned stuffing cubes ½ cup unsalted butter 1 ½ cups yellow onion, diced (from 1 large or 2 small onions) 1 cup celery, diced (from 3 large celery stalks) 4 garlic cloves, finely chopped 1 pound bulk sweet Italian sausage (casings removed) 2 ¾ cups low sodium chicken broth 1 large egg, beaten 1 tablespoon fresh rosemary, chopped 1 tablespoon fresh sage, chopped ¼ cup fresh parsley, chopped ½ teaspoon salt ½ teaspoon freshly ground black pepper