This light, spring salad includes speck, a flavorful smoked ham that's a specialty of the South Tyrolean region of Italy.
Bring vinegar and spring onion to a boil in a small saucepan over medium heat. Cook until liquid is reduced by half, about 2 minutes. Set aside.
Bring a medium saucepan of water to a boil; add salt. Prepare an ice-water bath, and set aside. Peel bottom 2 inches of asparagus, reserving peels. Blanch asparagus peels 2 minutes; immediately transfer with tongs to ice-water bath to stop the cooking. (Keep water at a boil.) Remove peels with a slotted spoon; set aside. (Reserve ice-water bath.) Blanch asparagus until crisp-tender, about 4 minutes. Immediately transfer with tongs to ice-water bath. Drain; set aside.
Cut lower third from each asparagus spear, and transfer pieces to a food processor. Set remaining asparagus aside. Add reserved peels, the vinegar mixture, and mustard to processor; process until smooth. With processor running, pour in lemon juice and then oil in a slow, steady stream. Pass mixture through a fine sieve into a small bowl; discard solids. Season vinaigrette with salt and pepper.
Put salad greens and mint in a large nonreactive bowl (glass or ceramic). Drizzle with vinaigrette, and gently toss. Divide asparagus among serving plates. Top each serving with dressed salad greens and a slice of speck.
You can use thinly sliced prosciutto or another cured ham instead. Spring onions are a more mature version of scallions; they have large bulbs, which can be purple or white, and are available at specialty-food stores and farmers' markets.