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 8 heaped tablespoons of the garlic mayonnaise (keep the leftovers in a jar in the fridge for up to 3 days.). 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!