Give your veg a good wash: potatoes, carrots, parsley root, and the bit of celeriac (if using). I cook them in their skins so they don’t drink water and go bland.
Pop them into a pot, cover with water, add a pinch of salt if you like, and simmer until just tender. You want a knife to go in easily, but the veg should still hold their shape (usually up to 30 minutes, depending on size).
Drain straight away and let everything cool enough to handle. Peel, then dice into small, even cubes.
At the same time, boil the eggs: start them in cold water, bring to a boil, then cook about 9 minutes for hard-boiled. Cool quickly in cold water, peel, and dice.
If you’re using leek, finely chop the white part (about 50 g). Put it in a sieve over the sink and pour boiling water over it from the kettle. This softens that raw bite without cooking it into mush. If you prefer onion, just finely chop a small one instead (or skip leek/onion completely).
Peel the pickles thinly, trim the ends, and dice them small.
Peel the apple, remove the core, and dice to match the pickle size. This is what keeps the salad lively and not just “mayo and beige.”
Tip everything into a big bowl: cooked veg, eggs, pickles, apple, leek/onion, and well-drained peas.
Add mayo + yoghurt, then a tiny bit of mustard (start small - you can always add more). Mix gently but properly, scraping from the bottom so the dressing coats everything.
Add salt and pepper, mix, then taste. Potatoes love to “steal” seasoning, so don’t be surprised if it needs a little more. Add in small pinches, tasting each time.
If you’ve got time, chill the salad for 1–2 hours before serving. The flavour settles, the potatoes absorb the dressing, and it tastes more “Christmas table” than “just mixed.” Smacznego!