Pre-heat your oven to 350 F.
In a pan over a medium heat add your butter. Once melted add together the ham, asparagus and onion then cook for around 3 minutes.
Mix together your eggs, heavy cream, mustard, salt and pepper in a bowl.
In a baking dish add the ham and asparagus mixture together then pour over the egg mixture. Bake for around 20 minutes.
Add the grated cheese over the top and bake for another 10 minutes or until the cheese is golden brown.
Let it cool slightly before slicing then serve straight away, delicious!