Heat the olive oil in a pan and gently cook the onion and garlic for 4-5 minutes without browning.
Stir in the flour and stir over a medium heat for 1 minute. Add the wine and stir until smooth. Simmer over a medium heat for 5 minutes stirring from time to time to keep the sauce smooth.
Stir in the cream and stock and bring to a simmer. Finally add the mussels and spinach (if using) and season to taste. Cook for 1 minute stirring from time to time to heat through.
Serve immediately stirred into your favourite cooked pasta.