Preheat oven to 400°F. Spray or rub oil onto a baking sheet.
Peel and cut potatoes into 1-inch chunks. Place in a large bowl with oil, smoked paprika, salt and pepper. Toss to coat well. Spread potato chunks evenly on baking sheet, making sure they’re all in one layer.
Roast in preheated oven 30 to 40 minutes, stirring after 15 minutes. Potatoes should be cooked through, but still firm and nicely browned. Remove from oven, stir again and set aside. (They will continue cooking for a few minutes as they cool.)
For dressing:
Combine all dressing ingredients in a large bowl and stir well.
For vegetables:
Chop celery, radishes, and cilantro, placing in the large bowl as you finish each one. Thinly slice scallions (both green and white parts). Add to bowl and toss.
Add cooled roasted potatoes and toss again until well mixed.
Chill at least 30 minutes. (Flavor improves if chilled overnight.) Will keep up to three days.