Spinachsoup

I got the recipe for this soup when I was in hospital, cause of a
burn out. 1 serving has only 40 calories, but I promise it is
delicious. I didn't believe it first, but it really is lol.
Ingredients for 4 serves:
1/2 Onion, chopped fine
4dl fat free broth
2dl Milk
2 Tablespoons of flour (don't know the proper English term, but you
have to brush over the spoon so the flour is not higher then the rim
of the spoon)
2 handful of fine chopped spinach (we get them as cubes, frozen and
of course unseasoned)
Salt and nutmeg
Preparation:
Steam the fine chopped onions in a pot with a bit of water, till
they start to smell good. Add the broth.
Mix the flour and milk together and then add slowly to the soup. Let
it cook, on little heat, for about 10 minutes.
Short before serving, add the frozen spinach to the soup and cook
again for another 5 minutes.
Season soup with salt and nutmeg.