In a medium bowl, whisk together the oil, mustard, vinegar and 1/4 teaspoon salt until smooth and emulsified.
Meanwhile, bring a large pot of water to a boil over high heat. Sprinkle generously with salt. Add the asparagus to the water and, depending on the size of the asparagus, blanch for 3 to 5 minutes. It should still have some bite left to it. Add the warm asparagus to the dressing and toss well to coat.
Fan the asparagus out on a platter and sprinkle with the mizuna. Scatter the hazelnuts and goat cheese on top and drizzle with any dressing that may remain in the bowl.