For the mousseline sauce: Place the egg yolks and citrus juices in a bowl over simmering water or in a double boiler. Cook, whisking, until thick and frothy with a ribbonlike texture, about 10 minutes. Remove from the heat and whisk in the butter to emulsify. Season with salt and pepper. Set aside.
For the leeks: Thinly slice the leeks into matchsticks. Add the butter to a large skillet over medium heat, and cook until brown, 5 to 6 minutes. Add the leeks and saute until translucent and soft, 2 to 3 minutes. Season with salt and pepper. Set aside.
For the asparagus: Bring a medium saucepan filled with cold water to a simmer, then add the salt. Cut the tough ends from the bottoms of the asparagus and peel the stalks. Blanch the asparagus in the simmering water for 3 to 4 minutes. Meanwhile, add ice to a medium mixing bowl filled with cold water. Add the blanched asparagus to the ice water to stop the cooking. Once the asparagus have chilled, remove from the ice water and pat dry.
Arrange the leeks on a serving platter. Lay the asparagus on top. Glaze the asparagus with the mousseline sauce. Serve with the remaining sauce on the side.