Place potatoes in a medium pot and cover with water by at least 1". Add ¼ cup salt and bring to a boil. Cook potatoes until tender while still holding their shape, about 10 minutes.
Meanwhile, in a small bowl, combine garlic, oil, mustard, pepper, ¼ cup vinegar, and 2 teaspoons salt.
Drain potatoes and spread on a parchment-lined baking sheet. Drizzle potatoes with remaining 2 tablespoons vinegar. Let cool 5 minutes.
Transfer potatoes to a large serving bowl. Add dressing, scallions, parsley, dill, and tarragon. Toss to combine; season to taste with salt and pepper, if needed. Enjoy at room temperature or cover and chill until ready to serve.
