Using a rolling pin, cut the cucumber in ½ and bash it, then cut it into segments.
Crush the walnuts and toast in a dry frying pan over medium heat until smelling aromatic, but don't burn.
Crumble the blue cheese in a large bowl. Add the mustard, honey, yoghurt, and the juice of 1 lemon—season with salt and pepper.
Toss the celery, cucumber, red onion, apple, and walnuts into the dressing until well coated. Season to taste with extra lemon, salt, and pepper if necessary.