This is really good! And endlessly versatile. You could add fresh or dried herbs; cayenne or chili powder;  curry powder; chopped tomatoes…Also, there are several different mustards you could use. Instead of lettuce: spinach, arugula, bok choi…

Here are two versions:

Version One:

3 c chopped lettuce–not too coarsely or too finely chopped
1/3 to 1/2 c mayonnaise
1/3 to 1/2 c mustard
salt and pepper to taste

1. Combine all ingredients. Chill until ready to serve.

Version Two:

3 c chopped lettuce–not too coarsely to too finely chopped
1/4 to 1/3 c mayonnaise
1/4 to 1/3 c mustard
1/4 to 1/3 ketchup or chili sauce

1. Combine all ingredients. Chill until ready to serve.