Cook the potatoes in boiling salted water for 20-30 minutes or until tender; drain. Rinse with cold water; cool completely. Cut into 1/4" slices; place in a large bowl. Add onions, parsley and 3 tbs. 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, pepper, cayenne pepper and remaining vinegar; bring to a boil, whisking constantly. Pour over the salad; toss gently. Serve immediately.