Gently boil the potatoes in salted water until tender. Drain well and leave to cool slightly.
Make the mayonnaise by whisking together the egg yolk and English mustard in a bowl. Then add the olive oil very gradually, drop by drop – once you’ve blended in a quarter of the oil, add the rest a little more quickly, whisking all the time. Pound the garlic with a pinch of salt in a pestle and mortar and add to the mayonnaise. Season with sea salt, freshly ground black pepper and a squeeze of lemon juice.
Slice the cooked potatoes in half and dress with the garlic mayonnaise. Turn the cress punnets on their sides and slice the cress stalks near the base. If the cress is a bit dirty, give it a good wash before folding into the dressed potatoes. Then tuck in and enjoy!