Put the potatoes in a pan of cold water, bring to the boil, then simmer for 12 minutes until tender. Add in the green beans for the last 3 minutes of the cooking time, then drain and put into in a large bowl.
In a small bowl, whisk together the crème fraîche, mayonnaise, red wine vinegar and Dijon mustard until combined. Set aside.
Add the spring onions, red onion, tarragon and goats’ cheese to the potatoes and beans. Drizzle over the dressing, mix gently to combine, and serve piled on a platter.
Cook’s tip: why not boil the potatoes and blanch the beans in the morning? Cool under cold running water, cover and chill until ready to assemble the salad.