Boil potatoes until soft, remove from water, and let cool completely. Slice potatoes into 1/4-inch slices. On a large serving platter, alternate sliced potatoes with sliced onion. In a small bowl, whisk together oil, vinegar, garlic, minced parsley, sugar, salt, and black pepper. Pour mixture over potatoes and onions then toss lightly. Season to taste with additional salt and pepper; refrigerate. For best flavor, refrigerate overnight.