Dressing can be made in advance. Store in the refrigerator for up to 3 weeks.
|Combine the mayonnaise, scallions, basil, lemon juice, garlic, anchovy paste, salt, and pepper in the food processor and blend.|
|Add the sour cream and process again until just blended.|
|Wash, process, and plate the lettuce. Cut the tomatoes into wedges and place on top of the lettuce. Top with dressing and serve.|