Rub and wash the potatoes thoroughly, then cook until tender in lightly salted water. This will take 10-15 minutes, depending on their size.
Slice the beans into three equal pieces, then add them to the potatoes for the last 3-4 minutes of the cooking.
Meanwhile mix the dressing sauce and season with salt and pepper. Slice the onions and chop the tarragon.
Drain the potatoes and green beans and put them into a serving dish. Add the onion and tarragon, then pour over the dressing.
Crumble the goat's cheese on top.
Serve it warm or cold.