Preheat the oven to 180ºC, fan 160ºC, gas 4. Bring a medium pan of water to the boil and blanch the asparagus spears for 3 minutes until just tender. Drain, cut into 3cm pieces and set aside.
Meanwhile, melt the butter over a medium heat. Add the leek and cook for 5 minutes until soft, then remove from the heat and set aside to cool.
In a large bowl, mix the leek and asparagus with the bread, Cheddar and ham. In a separate bowl, whisk together the remaining ingredients, then gently fold it into the leek mixture.
Cut squares of baking parchment with pinking shears and use to line a 12-hole muffin tin. Spoon in the mixture and cover the whole tray with clingfilm. Chill in the fridge for 30 minutes, then remove the clingfilm. Bake for 25-30 minutes until golden. Serve warm or cold.