Place sausage in a microwave-safe bowl. Microwave, uncovered, on high for 3 minutes. Drain and set aside.
Place butter in a 2-1/2-qt. microwave-safe dish. Heat on high for 45-60 seconds or until melted. Whisk in flour, salt and pepper until smooth. Gradually whisk in milk. Microwave, uncovered, on high for 8-10 minutes or until thickened and bubbly, stirring every 2 minutes.
Stir in potatoes and onion. Cover and microwave on high for 4 minutes; stir. Heat 4 minutes longer. Stir in the sausage. Cover and cook for 8-10 minutes, stirring every 4 minutes or until potatoes are tender and sausage is heated through. Stir. Let stand, covered, for 5 minutes. Sprinkle with parsley if desired.