Preheat the oven to 180°C/fan 160°C/gas mark 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 eggs, milk and mustard powder together in another bowl, then gently fold into the bread mix.
Line a 12-hole muffin tin with cases. 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.