Melt butter with oil in pan and sauté onions until soft but uncoloured.
Add shitake mushrooms and cook over a medium heat for a few minutes. Set aside a few for garnish before adding remaining mushrooms to pan. Season with a light pinch of salt, lashings of pepper and sauté for a couple of minutes.
Add boiling liquid along with dried mushrooms and return to boil. Strip off thyme leaves and add to pan.
Simmer for 20 minutes. Blend cornflour in a small cup of water and add to pan. Bring to boil, stirring, and simmer for a few more minutes to thoroughly cook flour. Taste and adjust seasoning.
Blend soup until smooth.
Stir in most of the cream and serve with a little swirl of remaining cream and a few shitake slices.