Put potatoes in a double boiler with water and steam for 10-15 minutes until just tender. Let cool slightly. Wrap each potato with half a slice of prosciutto and set on sheet pan. Drizzle with olive oil and roast for 10-20 minutes until golden and prosciutto is slightly crisp around edges.
While potatoes are cooking, make Dijon drizzle by combining all ingredients and whisking together. Taste and adjust seasoning if necessary. Spoon over potatoes and serve immediately. Serves 4-6.