Heat the oil in a large skillet over medium high heat. Add turkey sausage, potatoes, pepper, onion and garlic to the pan. Saute, stirring often until veggies are soft and potatoes and sausage have browned, 5-7 minutes.
Stir in half of mustard sauce and toss to combine with the sausage mixture. Add water to the skillet, reduce the heat to medium and cover and simmer for 5 minutes.
Serve in 4 bowls and top each dish with remaining mustard sauce. Garnish with more scallions. Serves 4.