Preheat oven to 400º F. Slice each potato into eight wedges, into four if potatoes are small. Toss with oil, sesame seeds and a few grindings of pepper. Arrange in a single layer on a baking sheet. Roast for 40 – 50 minutes, until they are crisp and brown, turning potatoes over with a spatula once in the middle of baking time.
While the potatoes roast, process the remaining ingredients in a food processor or blender until the sauce is silky and thickened. Drizzle in the water, one tablespoon at a time, and process again until the sauce reaches a smooth, gravy-like consistency.