Heat a large saucepan over a low heat and add the butter. When the butter has melted, add the onion and cook until soft and translucent.
Add the mushrooms, season with salt and pepper to taste, then increase the heat and stir. When the moisture from the mushrooms has been released, allow it to evaporate, concentrating the flavour. This might happen quickly or slowly, depending entirely on the type of mushrooms used.
Add teh paprika and cayenne and fry for a few moments longer, until the paprika turns a shade darker. Finally, add teh sour cream or creme fraiche and dill, and allow it to bubbvle gently for a further 5 minutes or so.