Melt butter in a large skillet. Add onions and celery, and cook over medium heat until onions are translucent, about 10 minutes. Add sage, stir to combine, and cook 3 to 4 minutes. Add 1/2 cup stock, and stir well. Cook for about 5 minutes, until liquid has reduced by half. Transfer onion mixture to a large mixing bowl. Add all remaining ingredients, including the remaining stock; mix to combine.