Prep: 1. Chop lettuce and tomato. Make: 1. Place 1/3 cup yogurt, scallions, basil, lemon juice, garlic, fish sauce, salt, and pepper in a blender and blend until smooth. 2. Add the remaining yogurt and process just until blended. (If not using immediately, refrigerate the dressing until ready to serve.) 3. Drizzle dressing over lettuce and tomato.
You can also use one head, or two hearts, of romaine instead of bibb lettuce if you like. For a tangier dressing use mayonnaise in place of the first 1/3 cup yogurt.