Discard any mussels that are broken or do not close immediately when sharply tapped with a knife. Put them in a large saucepan with the lager, cover and cook, shaking the pan frequently, for 4 minutes until all the shells have opened. Discard any that remain closed. Strain, reserve the cooking liquid and keep it warm.
Meanwhile, melt the butter in a large saucepan and fry the onion, garlic, ginger and curry powder, stirring frequently, for 5 minutes. Strain in the reserved mussel liquid and bring to a boil. Boil until reduced by half, whisk in the cream and lemon juice and simmer gently.
Stir in the mussels, warm through and season to taste. Garnish with chopped parsley and serve with crusty bread, if liked.