The dressing for this easy-to-assemble salad can be made up to three days in advance.
Make vinaigrette: In a small bowl, whisk together oil, lemon zest and juice, vinegar, and sugar; season with salt and pepper.
When ready to serve salad, place lettuce, radicchio, scallions, and tomatoes in a large bowl; add vinaigrette, and toss to combine. Serve immediately.