Mix the milk, eggs, grated cheeses and chopped herbs in a bowl. Season with salt and pepper.
Heat half of the oil in a large frying pan and fry the spring onion until soft, then add to the egg mixture.
Heat the remaining oil and pour in the egg mixture. Allow to cook at a low temperature until the surface is jelly-like. Garnish with a few herbs leaves, then put the cover on. Let it steam for a couple of minutes.
When the surface looks cooked through, transfer the frittata onto a plate.
Slice it like a cake or cut it into little, bite-size cubes. Serve it with rocket or watercress.