Inspect the mussels carefully, discarding any with cracked or broken shells. Tap any open mussels gently and discard those that do not close. Rinse under cold running water, scrub away debris and remove any remaining beards. Keep refrigerated until ready to cook.

Melt the butter in a large, heavy-bottomed pot over medium heat. Add the sliced leek and cook gently for 5 to 7 minutes, stirring occasionally, until softened but not browned. Add the garlic and thyme and cook for another 30 seconds.

Pour in the Scotch whisky, if using, followed by the vegetable or seafood stock. Add the black pepper and bring the liquid to a gentle simmer. For an alcohol-free version, replace the whisky with an equal quantity of additional stock.

Add the cleaned mussels to the pot and cover immediately with a well-fitting lid. Steam for approximately 5 to 9 minutes, shaking the pot gently once or twice, until the shells open and the mussels are cooked through. Discard any mussels that remain closed.

Remove the pot from the heat. Stir in the chopped parsley and lemon juice. Taste the broth and adjust the seasoning if needed. Divide the mussels between four warm bowls and spoon the buttery leek broth over the top. Serve immediately with crusty bread.