Thaw frozen spinach or steam the fresh spinach.  Let it cool a little then squeeze as much water as possible out of it.

While waiting for the spinach, whisk the eggs with the cheese, milk and salt and pepper. Add the cooled spinach and mix everything together.  Don't add the spinach while it is hot otherwise the egg will cook around it and make lumpy mess. :)

Heat the oil in a frying-pan on medium heat.  I use a 26cm (10inch) diameter pan. Pour in the frittata mixture. Put a lid on the pan and leave it, without shaking or stirring, to cook for 7-10 minutes until the top is set.

Now comes the tricky part, but take a deep breath and "just do it!" as those shoe people say!  Slide the frittata out of the pan onto the lid or onto a plate.  It needs to be flipped so that the part which is the top at present can brown a little.  My method is to tip the frying-pan upside down and place it on top of the frittata and then turn everything right-way up again.  If you're feeling adventurous, you could try and flip the frittata off the plate into the pan.  Just have plenty of cleaning supplies handy!

Let what is now the bottom of the frittata brown for 2 - 3 minutes, then slide onto a plate.