Heat oven to 350 degrees. Scrub and dry potatoes. Leave the smaller potatoes whole, and cut the larger potatoes in half. The objective is for all the potatoes to be about the same size for even cooking.
Drizzle the potatoes with 1 1/2 T olive oil and toss with your hands to evenly coat the potatoes in the oil. Season with salt and pepper to taste. Roast for 15 minutes, flip them over with a spatula and roast another 15 minutes.
For the dressing, put the shallot, mustard and vinegar in a small bowl and stir to combine. Gradually whisk in 1/3 cup olive oil to form an emulsion. Season with salt and pepper.
When potatoes are done, remove from the oven and transfer to a large bowl. Add celery, chives and mint. Toss with the dressing.