Cook potatoes until tender, drain.
Cool completely, dice into a large bowl. Add onions, parsley and 3 T. vinegar. Toss.
In a medium skillet, cook sausage in oil for 5-10 minutes or until it begins to brown. Remove with slotted spoon and add to potato mixture.
To drippings in skillet, add mustard, garlic, black and cayenne pepper and remaining vinegar; bring to a boil, whisking constantly. Cook until blended and slightly thickened.
Pour over potatoes and toss gently. Serve
immediately.