In a large saucepan, place the sausage and enough cold water to cover by 2 inches. Bring to a gentle simmer and cook until tender and cooked through, about 30 minutes. Do not let boil. Remove the pan from the heat and let the sausage sit in the hot liquid for 5 minutes. Transfer the sausage to a platter to cool slightly.
Meanwhile, in another large saucepan, place the potatoes, kosher salt, and enough water to cover by 2 inches. Bring to a boil. Reduce the heat to a low boil and cook until the potatoes are tender yet still firm, 15 to 20 minutes. Drain and let cool slightly. Slice into 1/4-inch thick rounds and place in a large salad bowl. Sprinkle with the white wine while still warm.
In a mixing bowl, whisk together the Dijon mustard, white wine vinegar, shallots, garlic, salt, and pepper. While whisking, add the oil in a steady stream and whisk until the vinaigrette is thick and emulsified. Adjust the seasoning, to taste. Pour over the potatoes and gently toss to coat.
Slice the sausage into 1/4-inch thick rounds and add to the potato mixture. Add the chopped parsley and pistachios, and toss gently to combine. Garnish with chervil or baby parsley sprigs and serve warm.