Slice the 3 leeks into thin half-moons. Use only the white and pale green parts.
Add the leeks to the boiling water and blanch for 1 to 2 minutes.
Drain immediately, then rinse under cold water or shock in an ice bath.
Pat the leeks very dry. This matters. Wet leeks will dilute the dressing.
While the leeks dry, whisk together the 150 g full-fat Greek yogurt or sour cream, 1 tbsp wholegrain mustard, 1 tbsp lemon juice, 1-2 tsp horseradish, salt and white pepper to taste, and 1/4 teaspoon celery seed.
Add the leeks, 1 apple (cut into matchsticks), 50 g walnuts (toasted), and 2-3 tbsp dill (chopped) to a large bowl.
Spoon in the dressing and toss gently until everything is coated.
Taste and adjust the seasoning. Add a little more salt, pepper, or lemon if needed.
Chill for 10 minutes before serving so the flavors can settle.
Notes
Dry the leeks completely - wet leeks will water down the dressing, and you'll lose all that creamy texture.Blanch, don't overcook - one to two minutes in well-salted boiling water is all you need.Cut the apple last and toss immediately