The scarlet onions and fine slivers of orange peel in this green sauce look and taste like spring. The sauce can be made ahead of time, the orange juice and vinegar added at the very end so that the greens remain bright and snappy. This sauce is also good over grilled asparagus. Serve at room temperature or slightly chilled.
Trim the asparagus by snapping off the woody, tough ends, and peel thick asparagus. It will look and taste better. Bring a quart or two of water to a boil in a wide pan, generously salt to taste.
FOR THE PICKLED RED ONION: While the water is coming to a boil, pickle the red onion and make the sauce. To make the pickled red onion, peel a whole, small red onion, then slice it thinly into rounds. Separate the rounds and put them in a bowl, then cover with a white wine or rice wine vinegar and a few pinches salt. Toss them with your finger so that they are all coated, then refrigerate until you need them. It takes only about 10 to 15 minutes for the color to blossom.
FOR THE ORANGE ZEST: Bring a small saucepan of water to a boil. Zest a few strips of orange peel in wide bands. Julienne the bands into thin strips and blanch in boiling water for 10 to 15 seconds. Drain and chop half of the orange rinds into a fine dice. Set aside.
Add the trimmed, peeled (if necessary) asparagus to the wide pan of salted boiling water, and cook until it is tender and bright green, about 5 minutes for thin asparagus, 7 to 8 minutes if it is thick. Set the cooked asparagus on a towel to drain briefly.
FOR THE SAUCE: Meanwhile, combine the chopped parsley and capers in a bowl. Mince half the slivered orange zest and add it to the bowl along with half of the pickled onion, minced. Stir in the mustard, orange juice, vinegar, and olive oil. Season with salt and pepper.
TO FINISH: Drain the asparagus. Lay the asparagus on a platter. Ladle the sauce over the stems and garnish with the remaining slivers of orange peel and pickled onion.Suggestions: You can vary the taste by using aromatic sesame or peanut oils in place of the olive oil. Both flavors go well with asparagus and also harmonize with orange. Keep the parsley or use cilantro instead if you're wild for that herb. Other spring herbs to consider including are tarragon, lovage, chives and chervil and all are excellent with asparagus.