For the asparagus:
1) Preheat the oven to 200 degrees C.
2) 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 tray, in a single layer, and roast until tender but still firm and moist, about 10 minutes.
3) Transfer the asparagus to a serving platter, toss with the vinaigrette and serve. The dish may be eaten warm or cold.
For the vinaigrette:
1) 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.