Slice the potatoes and cook them in a large pan of salted boiling water for 7 minutes. While they’re cooking, trim and diagonally slice the runner beans and chop the spring onions. Add the beans to the pan and cook for a further 7-9 minutes, until the potatoes and beans are just tender.
Drain the veg and run under cold water to halt the cooking. Shake the colander to get rid of as much water as possible, then tip into a large bowl. Add the onions and tomatoes, crumble in the cheese and mix well.
Add most of the mint and dressing and toss everything lightly together. Tip into a serving dish, drizzle with a little more dressing and scatter over the rest of the mint.