Preheat oven to 500 degrees. Line a baking sheet with foil. In a large bowl, toss the potatoes, oil, and next 3 ingredients, then arrange on baking sheet in a single layer. Roast about 25 min. While potatoes cook, combine all the ingredients for the Cilantro Sour Cream, mixing well. Chill until potatoes are tender. Serve in a bowl next to Cumin Potatoes for dipping or dollop on top.