Even though the basic concept is hundreds of years old, perfectly cooked asparagus drizzled with a light, tangy sauce still pleases - especially when you start with fresh, tender asparagus. The "vinaigrette" in this recipe deviates from the classic version. Here it's simply a purée of asparagus seasoned with some lemon juice and mustard. The sauce is delicious on its own and can also be used as a dip or sandwich spread.
Snap off the tough ends of the asparagus stalks and place half of the ends in a small pan; discard the remainder. Peel 4 asparagus spears and then cut them into 1/2-inch pieces; add these to the pan. Add 1/4 cup of water and 3 tablespoons of the oil to the asparagus and bring to a simmer over low heat. Cover the pan and cook for 5 minutes. Then, uncover the pan and continue cooking until the asparagus is very tender, the water has evaporated, and only oil is left, 3 to 7 minutes, depending on the thickness of the vegetable.
Transfer the mixture to a blender and purée until completely smooth. Place 3 tablespoons of the purée in a small bowl. Add the mustard, lemon juice, sugar, salt, and pepper; stir to combine. Add 1 to 2 tablespoons of the oil to thin the sauce to the desired consistency. Adjust seasonings as needed.
Meanwhile, peel the remaining asparagus stalks. Steam the asparagus until crisp-tender, 3 to 6 minutes. Immediately plunge the asparagus in a bowl of ice water to stop the cooking process. When cool, drain thoroughly and dry the spears with paper towels.
To serve, divide the asparagus between 2 plates and spoon some of the vinaigrette sauce over the spears. Serve hot or at room temperature for the best flavor.