1. In a large pot, melt the butter. Add the onions and cook for 2 minutes over low heat.
2. Add the potatoes, cover the pot and let the vegetables sweat for 4 minutes.
3. Add the broth and season with salt and pepper. Continue cooking for 30 minutes covered.
4. Using an immersion blender, reduce the soup to a fine puree. Add the cream and mix. Put the pot back on the heat for 3 minutes to reheat.
5. Chop the basil and parsley finely with a knife. Add the chopped herbs to the soup just before serving.