1) Place the mayonnaise, spring onions, basil, lemon juice, garlic, anchovy paste, salt and pepper in a blender and blend until smooth. Add the sour cream and process just until blended. (If not using immediately, refrigerate the dressing until ready to serve.)
2) Cut each head of lettuce into quarters, remove some of the cores, and arrange on 6 salad plates. Cut the tomatoes into wedges and add to the plates. Pour on the dressing and serve.