Preheat oven to 350 degrees. In a small skillet, saute chopped onion until slightly brown and translucent. Set aside. In a large mixing bowl, combine sausage, biscuit mix, cheese, mustard, black pepper, sag (if desired), and onions. Form into walnut sized balls and place roughly 1 inch apart on cookie sheets lined with foil. Bake in the preheated oven for 20 minutes, or until sausage is browned and cooked all the way through. Note: The sage is entirely optional. I like to add the sage if I make these during Thanksgiving or Christmas because it just gives it a "taste of the holidays."