Make the quick-pickled red onion by adding the sliced red onion, cider vinegar, salt and water to a bowl. Stir, then leave to one side.
Boil the potatoes with some salt for 20 minutes. After 15 minutes, add the green beans in with the potatoes and cook for the remaining 5 minutes.
Once the potatoes and green beans are cooked, drain and rinse under cold water to cool them down and stop them from cooking.
Place the green beans, potatoes into a large serving bowl, and using a fork, lightly press into the potatoes to smash them.
Add to the bowl the quick pickled red onions and all the other ingredients and gently mix.