Image credit to: Boston Organics
In small nonreactive bowl, whisk together vinegar, cornichons, 1/4 cup minced dill, shallot, mustard, sugar and 1/2 tsp salt until sugar dissolves. Slowly whisk in olive oil until well blended to make a vinaigrette. Taste and adjust the seasoning with salt.
In large saucepan, combine potatoes and 1 Tbsp salt, add water to cover potatoes by 1 inch and bring to a boil over high heat. Reduce heat to medium, cover partially and simmer until potatoes are just tender when pierced with a knife, 7 to 9 minutes. Fill large bowl two-thirds full with ice water. Drain potatoes in colander and immediately transfer to ice water. Let stand until cool, then drain again.
Whisk vinaigrette to recombine, drizzle over potatoes. Add celery and radishes and stir gently. Taste and adjust seasoning with salt.
Transfer the salad to a serving bowl and sprinkle with the 2 Tbsp chopped dill. Serve immediately, passing the crème fraîche for dolloping at the table.