fresh asparagus (about 12 spears)
extra-virgin olive oil (divided)
fresh lemon juice (preferably Meyer)
Freshly ground black pepper (to taste)
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.