1. Fill a large pot with about 2 inches water and add a pinch of salt. Set steamer basket in pot; make sure water doesn't seep through holes. Bring to a boil, and then reduce to a rapid simmer.

2. Add potatoes to steamer basket. Cover and steam until tender when pierced with the tip of a paring knife, 20 to 25 minutes. Remove potatoes and allow to cool slightly. Peel potatoes, and slice into 1/4-inch-thick slices.

3. Combine onion, vinegar, sugar, salt, and pepper in a large bowl. Add potatoes and use a large rubber spatula to gently combine, trying not to break up potato slices. Let stand for 10 minutes. Add mayonnaise, buttermilk, and cornichons. Add mixture to another bowl and repeat process of transferring potato mixture from one bowl to another to mix gently until well combined and creamy. Refrigerate for at least 2 hours; the mixture will thicken as it stands. Garnish with parsley and/or sliced hard-boiled eggs, if desired. Season with salt and white pepper.