Mussel, Leek and Tomato Casserole, with Spinach and a Warm Poached Egg

Mussels have been eaten since prehistoric times in Britain, and a number of recipes for them exist from the fourteenth century onwards. In the very early days, they used to be baked on hot stones beside the sea, and a little butter was inserted as the shells opened. They were also cooked in pies, boiled and served with parsley, or packed into rolls or baked potatoes in Ireland.

This is quite a simple dish, with the mussel cooking liquor becoming the base to the sauce. Once everythin