Wash nettles and pluck the leaves. Peel and finely chop the onions.
Melt the butter in the saucepan, sauté the onion until translucent and dust with flour, sauté briefly and add the water. Bring to the boil and add the broth and nettles. Simmer for 10-15 minutes, season with salt and pepper.
Serve on plates and add the sour cream if you like.