In a large skillet over high heat, bring 1 inch of water to boil. Add asparagus and blanch 3 to 4 minutes or until crisp tender. Place asparagus in ice bath 5 minutes, then drain well. Cut into 2-inch pieces and place in medium bowl. Add radishes, scallions, and lemon zest; set aside.
In a small bowl, whisk lemon juice and sugar until sugar dissolves. Add oil and salt; mix well. Pour lemon juice mixture over vegetables; toss until evenly coated. Cover and refrigerate 30 minutes or until chilled.