Preheat the oven to 180°C, fan 160°C, gas 4.
Melt the butter over a medium heat. Add the leek and cook until soft, then allow to cool.
In a large bowl, mix the cooked leek with the bread, cheese, asparagus and ham. Whisk the remaining ingredients together in another bowl, then gently fold into the bread mix.
Cut squares of baking parchment (with pinking shears if you have them, for a decorative effect) and use to line a 12-hole muffin tin. Spoon in the strata mixture, then cover with tray with clingfilm. Chill for 30 minutes.
Remove the clingfilm and bake the strata for 25-30 minutes until golden. Serve warm or cold.
Cook’s tip: You don’t have to make your own pretty parchment cases – muffin or cupcake cases work just as well.