Slice the mushrooms and onions thinly.
Take a large heavy-based saucepan and gently heat the oil.
Gently fry the fresh mushrooms, onions and frozen mushrooms for 5 min.
Pour all the wine into the pan and let it boil until the wine has reduced to almost nothing.
Add the water to the pan.
Now add the dried mushrooms and stir.
Add the mushroom powder into the pan and let the mixture boil for 20 min.
Now add the cream into the pot and let it simmer for 60 min.
In a small pan place 100 g of butter, heat gently and let it melt, then add flour into it and combine it together to make a roux. Whisk the roux mixture into the soup and continue whisking until the soup thickens.
Let the soup simmer for 10 min and then add the remaining 200 g of the butter.
Blitz the soup with a hand-held blender.
Season with salt and pepper, also it is very good to add a little bit of sherry vinegar at the end.