Using a box grater, grate the cucumber into a sieve and set over a medium-sized bowl. Add a generous pinch of salt, toss well using your hands, then leave to drain whilst you prepare the rest of the ingredients (before using them later on, remember to give them a really good squeeze to extract as much of the water as possible).
Meanwhile, prepare the dressing: in a small bowl, mix together the tahini, crispy chilli oil, soy sauce, rice wine vinegar and runny honey. Season to taste with salt. Add a little of the drained cucumber water (waste not, want not!), then whisk until you have a silky smooth consistency. Set aside.
Boil the kettle, then pop the udon noodles in a large saucepan and pour over the boiling water. Cover with a lid and leave to stand for 5 minutes - or cook according to packet instructions. Drain well.
To assemble: place the drained noodles into a large bowl and pour over the dressing. Toss well to coat. Add the spring onions and shredded chicken, then toss again.
Spoon into bowls, drizzle with more chilli oil, if wished, sprinkle with toasted sesame seeds and enjoy!