Sauté or roast sausage until firm, browned, and cooked through, turning occasionally.
Divide the mashed potatoes among four plates. Place the mashed potatoes in the center of the plate.
Place one link of sausage in the middle of the mashed potatoes and about one teaspoon of grain mustard across the center of each link.
Spoon a little gravy or au jus around the mashed potatoes and serve with extra grain mustard.