Heat the oven. Preheat your oven to 425°F. Line a baking sheet with parchment paper so it is ready for the potatoes.
Coat the potatoes. Put the halved baby red potatoes into a medium bowl and toss them with 2 teaspoons olive oil until lightly coated.
Begin roasting. Arrange the potatoes evenly over the prepared baking sheet. Bake them in the preheated oven for 15 minutes.
Turn and finish roasting. Turn the potatoes, then return them to the oven for about 10 minutes more, until they are light golden and tender.
Cool the potatoes. Remove the roasted potatoes from the oven and leave them until they have cooled to room temperature.
Whisk the dressing. Meanwhile, use a large bowl to whisk together the remaining 1/4 cup olive oil, 1 1/2 teaspoons Dijon mustard and 1 tablespoon white vinegar. Season with kosher salt and black pepper.
Assemble the salad. Add the cooled potatoes to the dressing along with the chopped scallions, sliced celery, roughly chopped cooked bacon, finely chopped fresh tarragon and finely chopped fresh parsley.
Toss and serve. Gently toss everything until the potatoes and smaller ingredients are coated and evenly combined. Serve immediately, or refrigerate until you are ready to serve. Let the salad come to room temperature before serving.