Put about 1 litre (35 fl oz/4 cups) of water in a large pan, add a little salt and bring to the boil. Add the spinach, return to the boil and simmer for just about a minute, until the spinach has wilted. Drain, reserving the water.
Put the olive oil and garlic cloves in a large frying pan over medium heat. When you can smell the garlic, add the spinach, turning it over with a wooden spoon to collect the oil. Add the feta and 125 ml(4 fl oz/½ cup) of the spinach water. Mix the feta through, mashing it in with the spoon so it dissolves. Still at medium heat, sauté for a couple of minutes until everything has blended together and there is only a bit of liquid left in the pan. Remove from the heat, stir in the parmesan and taste for salt. Serve immediately, with some black pepper for adults only.