Cook the potatoes in a large pot until tender but firm, about 10-15 minutes. Drain and let them cool slightly
Prepare the vinaigrette by whisking together olive oil, red wine vinegar, Dijon mustard, grainy mustard, minced garlic, cornichon juice, salt, and pepper
Combine the slightly cooled potatoes with parsley, dill, green onions, shallot, and cornichons in a mixing bowl
Drizzle the vinaigrette over the potato mixture and toss gently to coat all ingredients
Adjust seasoning if necessary and serve warm or chilled. If serving chilled, let the salad marinate in the refrigerator for at least an hour
Garnish with fresh herbs before serving