Cut the pickles in very small cubes and use a sieve to drain very well.
Drain the peas then wash in cold water and drain again very well.
Wash potatoes and boil in skin until soft but still firm enough to cut with a knife without breaking.
Boil chicken breast, carrots and celery root if you opted until soft (you can use the broth to make a soup afterwards)
When all veggies and the meat are done, peel potatoes and then cut all veggies but peas in medium small cubes with a sharp knife.
Combine the boiled veggies, meat, peas and pickles and mix well but gently, using 2 spatulas.
Add mustard, salt and pepper to taste.
Make the mayonnaise: Grate the boiled yolk and mix with the raw one. Add mustard and mix well with a wooden spoon until smooth. Add oil bit by bit and mix continuously until well incorporated. Do not add more oil until the previous batch has been completely incorporated. Do not drop a lot or all of the oil at one time! If at any point the texture changes add a bit more mustard and/or lemon juice. When the mayo is done, add a tablespoon or two of lemon juice; this will make it taste a bit lighter.
Add mayo to the meat veggie mixture and mix well but gently using the 2 spatulas, until all ingredients are well coated in mayo.
Place the salad on a big plate or individual ones and decorate with more mayo, the boiled egg white (if your kids didn't eat it already lol) and pickles, cut in slices or cubes, your choice. You could also add some sliced olives to this decor.