Preheat the oven to 425°F. Halve the baby yellow potatoes (or quarter them if they are large). Toss the potatoes in 3 tablespoons of extra-virgin olive oil and 3/4 teaspoon of salt on a large baking sheet until they are evenly coated. Arrange the potatoes cut-side down and spread them out in an even layer. Roast them for about 25 minutes until they are golden brown on the bottoms. Flip the potatoes using a spatula or tongs and roast them for an additional 10 minutes until they are tender and lightly golden all over. Let the potatoes cool slightly on the baking sheet for 5 to 10 minutes.
In a large bowl, whisk together 1/3 cup of whole-milk plain strained (Greek-style) yogurt, 1/3 cup of mayonnaise, 1/4 cup of finely chopped red onion, 1/4 cup of finely chopped mixed tender herbs (such as tarragon, dill, and parsley), 1 small clove of grated garlic, 2 teaspoons of country-style Dijon mustard, 2 teaspoons of white-wine vinegar or Champagne vinegar, 1/4 teaspoon of ground pepper, and the remaining 1/4 teaspoon of salt until well combined. Stir the potatoes into the yogurt mixture until they are evenly coated. Transfer the mixture to a serving dish and garnish with additional herbs, if desired. This dish can be made ahead of time and refrigerated in an airtight container for up to 4 days.