Method:
In a small bowl, whisk together Dijon and lemon juice. Drizzle in oil while whisking constantly and then season with salt and cayenne; set dressing aside.
Bring a large pot of salted water to a boil. Add asparagus and simmer until just tender, 2 to 3 minutes. Using a slotted spoon, transfer to a large bowl of ice water to stop the cooking; drain well and transfer to a large bowl. Return water to a boil, add potatoes and simmer until just tender, 15 to 20 minutes; drain well and add to bowl with asparagus. Add dressing, chives and salt and toss to combine. Transfer to a platter and serve at room temperature.