Pick through the mussels, discarding any that are broken or that fail to close when tapped on the side of the sink. Steam the mussels open in a big pan with a lid on, adding a small splash of water first to get them going. (Cooking them in a large pan will help them open faster and not leave you with half open and half closed.) Cook the mussels for approximately 3 minutes until they have just presented themselves and can be lifted quivering from the shell. Collect the juice in a small bowl. Discard any mussels that have failed to open.
Melt the butter in a small saucepan, then scatter in the flour. Whisk it into the butter well, then cook gently for 30 seconds, or until the mixture foams a little. Add the mussel stock bit by bit. The mixture will clag up at first, then loosen as you add more liquid. When the mussel stock is gone, start adding the milk, bit by bit. When all the milk has been stirred in, let the sauce gently simmer for 7 minutes or so, stirring occasionally.
Toast the bread, then butter it and put it on two plates. Grate a little nutmeg into the white sauce followed by a grinding of black pepper. Lastly, add the mussels and parsley, season with a little salt if necessary and cook for a further 30 seconds until the mussels are hot. Spoon them on to the toast and eat accompanied with a mug of tea.