This is one of my favourites. A quick and easy way to use asparagus when it's plentiful and in season. The most important thing is to cook the frittata slowly.
Parmesan cheese is not truly vegetarian, as it contains animal rennet. To make this dish 100% vegetarian, omit the cheese or find a suitable vegetarian substitute made without animal rennet. In supermarkets look for the 'parmesan style hard cheeses' which are suitable for vegetarians.