Wash the mussels under cold, running water. Discard any open ones that won’t close when lightly squeezed.
Soften the onion and garlic in the butter over medium to low heat. Add the thyme, bay leaf and wine, and increase the heat to high. Add the mussels, cover with the lid and steam for 6 to 10 minutes until the mussels have opened.
Remove and discard the thyme sprigs, bay leaf and any unopened mussels.
Stir in the cream and chopped parsley, and season well.
Remove from the heat, spoon into four large warmed bowls and serve with lots of crusty bread.