Peel the cucumber, then halve it lengthways and scoop out the seeds. Cut into 1cm-thick half-moons and place in a large bowl along with the red onion. Sprinkle over two teaspoons of salt and tumble together.
Leave this to sit for 45 minutes to an hour, then rinse the cucumber and onion in plenty of fresh cold water and allow to drain.
Place the salted vegetables in a clean bowl with the vinegar, sugar, crushed coriander seeds, plenty of black pepper and half of the dill. Stir together and set aside while you cook the potatoes.
Place the potatoes in a large pan. Cover generously with cold water and set over a high heat. Add a tablespoon of salt and bring the water to the boil. Cook the potatoes for 8-15 minutes (cooking time will vary according to the variety and freshness of the potatoes), until tender.
Drain the potatoes, then return them to the pan. Add the garlic, olive oil and plenty of salt and pepper, and tumble the spuds well to combine. Leave to cool.
Lift the pickled cucumber and onion out of the vinegar and combine them with the cooled potatoes. Carefully toss through the pea shoots, if using, then scatter over the remaining dill and serve.