Gently scrub 350g of new potatoes, removing any flaky skins with your thumb.Boil them in ca 20 minutes, depending on their size. Test them with a skewer – it should slide through effortlessly.
Drain the potatoes, cut each one in half and put them in a large bowl.
While the potatoes are cooking, make the dressing. Put 2 tsp of mustard into a small bowl, add 2 egg yolks, a pinch of salt, then slowly beat in 150ml of groundnut or sunflower oil and 100ml of olive oil. Do this drop by drop at first, then quickening to a steady stream, until you have a thick mayonnaise. Mix in enough warm water to loosen the consistency so it will coat the back of a spoon.
Roughly tear 5g each of basiland parsley leaves and stir into the mayonnaise, then check the seasoning
Drop the potatoes into the mayonnaise while they are still warm. Spoon the dressing over the potatoes and leave for 10 minutes.
Tear 250g of smoked salmon into large pieces and add to the potatoes and 4 finely sliced radishes
(You could, of course, use a good-quality commercial mayo instead of making it yourself. Perk up the store-bought stuff with a shot of lemon juice, or better still, a tablespoon of juice from the pickle jar.)