Heat a large pan with a lid over a high heat until hot. Add the mussels and the white wine and place the lid on the saucepan. Cook over a high heat, shaking the pan regularly, until all the mussels have opened. Discard any mussels that have not opened after five minutes.
Drain the mussels into a large colander over a bowl to catch the cooking liquid. Allow to cool slightly.
Once cool enough to handle, remove the mussels from their shells.
In another pan, melt the butter, then add the onions and fry gently until softened but not coloured.
Add the flour and curry powder, stir well and cook gently for a 3-4 minutes.
Add the reserved cooking liquid and stir well. Bring to the boil, then reduce the heat to simmer for ten minutes.
Add the cream and bring back to the boil.
Add the mussels and stir well.
Just before you are ready to serve add the spinach and stir in to wilt down.
To serve, ladle the mussel velouté into warmed bowls. Sprinkle liberally with chopped chives and serve with crusty bread alongside.