Trim the roots and tough tops off the leeks, leaving about 6 inches of white and pale green; rinse them very well to remove any grit between the leaves. Scrape the baby carrots. Cut the bottoms off the asparagus to leave the tips about 4-inches long. Trim the tops off the beets, leaving about 1-inch of stem. Prepare an ice bath set aside. In a large pot of boiling salted water over high heat, cook the peas for about 2 minutes, until they are bright green and tender. Immediately remove with a skimmer or large slotted spoon and transfer to a bowl of ice water. Lift out and drain well; set aside. Repeat with the remaining vegetables. Cook asparagus until crisp-tender, 3 to 5 minutes; the leeks until just tender, about 3 minutes; the baby carrots until barely tender, 3 to 4 minutes; and the baby beets until tender, 20 to 30 minutes. Peel the beets (the skins will slide off) and quarter them. Rinse the mache well in a generous amount of cold water. Dry in a salad spinner; set aside. Peel the kohlrabi, and cut into thin julienne strips on a Japanese mandoline or by hand. Remove stalks and outer leaves from fennel and shave finely using mandoline or a sharp knife. Assemble the salad: Toss the mache with 1/4 of the vinaigrette dressing and mound it on a large platter. Toss the peas, beans, carrots, asparagus, kohlrabi and 2/3 of the sorrel with 2/3 of the remaining dressing. Arrange the mixed vegetables over the mache. Toss the beets with the remaining dressing and scatter them on top. Garnish with the remaining sorrel.
In a small bowl, combine the grapefruit, lime, and lemon juices. Sprinkle in the sugar and whisk until it dissolves. Gradually add the olive oil, whisking vigorously to create an emulsion. Season with salt and pepper. Yield: about 1 cup
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