Make vinaigrette: In a small bowl, whisk together oil, lemon zest and juice, vinegar, and sugar; season with salt and pepper. (To store, refrigerate up to 3 days.)
When ready to serve salad, place lettuce, radicchio, scallions, and tomatoes in a large bowl; add vinaigrette, and toss to combine. Serve immediately.
I made the Tangy Vinaigrette tonight to serve on a randomly-tossed green salad. Very good! This vinaigrette will be going into dressing rotation!