From "The Lighthearted Cookbook", by Anne Lindsay

1 cup buttermilk
1/3 cup light mayonnaise
one small clove garlic minced
1/2 - 1 tsp granulated sugar
1/4 tsp dry mustard
pinch freshly ground pepper
2 TB chopped fresh parsley
2 TB chopped green onions or chives, or to taste.

In a small bowl or jar, combine all ingredients and mix well.  (My immersion blender comes with a little chopper container and I sometimes make the dressing in that.) Cover and refrigerate for up to four days. Makes approximately 1 1/3 cups.

Variations that make it less "ranch-style" but result in a creamy dressing suggest replacing the green onions/chives with: 1/2 tsp dried dillweed; or 2 TB crumbled blue cheese; or 1/2 tsp dried ground cumin; or 1 teaspoon curry powder; or 2 TB tomato paste.

[Supposedly, the original Hidden Valley brand recipe is revealed here.]

[Related Blog post.]