1 teaspoon olive oil
1 lb. Italian sausage, casings removed
2 Tablespoons unsalted butter
2 stalks celery, thinly sliced
1/2 yellow onion, chopped
1 tart apple (such as Granny Smith or Pink Lady), cored and chopped
1/2 teaspoon each: dried sage, kosher salt and black pepper
1 1/2 cups low-sodium chicken broth
1 large egg, beaten
3 cups cubed toasted bread or dry stuffing mix