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 in a large 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 goat's cheese to the potatoes and beans. Drizzle over the dressing and mix gently before serving.