Combine the tomatoes, cucumbers, scallions, bell peppers, and lettuce in a large mixing bowl.
In another bowl, whisk the lemon juice, olive oil, tahini, vinegar, salt, and pepper together until the dressing is creamy and pale. If the dressing is too thick, gradually and a little water, whisking, until it reaches the desired consistency. If it is too thin, gradually add more tahini.
Just before serving, pour the dressing over the vegetables and toss thoroughly to coat. Serve immediately, to prevent the salad from becoming soggy.