Melt 50g of butter in a wide pan with a lid. Add 2 finely sliced medium leeks, white parts only, and 1 finely chopped garlic clove. Cook gently until soft.
Pour in 300ml of dry white wine and bring to a boil. Add 2kg of cleaned mussels, cover, and cook for 2-3 minutes, shaking the pan, until the shells open. Discard any that stay closed.
Stir in 150ml of heavy cream, season, and heat briefly. Scatter over 1 handful of chopped flat-leaf parsley.
