Preheat the oven to 200°C/400°F/Gas mark 6.
In a heavy saucepan, melt the low-fat spread or butter.
Add the thinly sliced onions and cook gently, covered, for 10 minutes, stirring occasionally, until soft.
Add the vegetable stock and season with a little salt and pepper.
Bring to a boil, reduce the heat, and simmer gently for 30 minutes.
Stir in the skimmed milk and heat through.
Whisk the egg yolks with the lemon juice.
Add 2 ladlefuls of the hot soup and whisk well.
Stir the egg and lemon mixture into the soup and heat through gently, still stirring, until slightly thickened.
Avoid boiling to prevent curdling.
To make the croûtons, spread out the bread cubes on a baking tray and bake in the oven for 10 minutes until a deep golden brown.
Ladle the soup into warm individual bowls and sprinkle with the croûtons just before serving.