Cookbook challenge, week 26: Potato salad with green goddess dressing

The theme for week 26 was green, and I found this recipe in Nigella Lawson's How to Eat. The original recipe uses 'little gems', which I think are baby cos lettuce. I just made it as a simple potato salad served on cos leaves.

Ingredients:
800g salad potatoes, cleaned and cut into 2cm square pieces
4 anchovy fillets, drained and finely chopped
2 tbs milk
2 tbs tarragon vinegar
6 tbs creme fraiche
8-10 tbs extra virgin olive oil
1 spring onion, finely diced,
3 tbs chopped tarragon
4 tbs chopped parsley
18 cornichons, or sliced gherkins

Boil the potatoes until just cooked, then drain and cool slightly under cold water.
Grind the anchovies using a mortar and pestle.
In a bowl, whisk together the anchovies, milk, vinegar and creme until smooth.
Slowly add the olive oil, whisking constantly.
Stir in the spring onion, tarragon, parsley and diced cornichons.
Check for seasoning, add black pepper.
Mix into the cooked potatoes.
Line a bowl with cos leaves, and add the potatoes.

It's pretty tasty - the dressing is really nice. I think if I make it again I'll add some bacon or prosciutto as it would be even better with that meatiness.