Preheat the oven to 400 degrees F.
In a large bowl toss the asparagus in the olive oil, and liberally season with salt and pepper. Spread the stalks out on a baking sheet, in a single layer, and roast until tender but still firm and moist, about 10 minutes.
Meanwhile, make the vinaigrette: In a small bowl, vigorously whisk together the mustard and lemon juice. Slowly drizzle in the olive oil, whisking quickly to emulsify the olive oil into the juice mixture. Season with salt and pepper, to taste.
Transfer the asparagus to a serving platter, toss with the vinaigrette and serve. The dish may be eaten warm or cold.