Tart cornichons and rich creme fraiche help flavor this French-inspired version of an old-fashioned favorite salad dressing.
Place potatoes in a large stockpot; fill with enough cold water to cover by 3 inches. Add salt; bring to a boil. Reduce heat, and let simmer until potatoes are just tender when poked with a fork, 40 to 45 minutes.
Remove from heat, drain, and sprinkle with vermouth. Let stand until cool.
In a medium bowl, whisk together the vinegar, sugar, creme fraiche, and celery seeds. Season to taste with salt and pepper.
Cut potatoes into bite-size pieces, and transfer to a serving bowl. Pour dressing over the potatoes. Add chopped cornichons, scallions, and parsley; toss to combine. Adjust seasoning with salt and pepper, and serve at room temperature.
The dressing can be made up to two days ahead and stored, refrigerated, in an airtight container.