Warm the oil in a large saucepan or stock pot over a medium heat. Add the onions and cook and stir until soft.
Combine the stock in a jug then pour over the onions and reduce the heat. Refill the jug with water and pour into pan. Once the onions are soft turn down the heat. Stir and sprinkle in the bread. Place lid on pan, turn up the heat until bubbling; stir regularly.
Once bubbling, turn down the heat and carefully add spinach. Replace the lid and turn up the heat until bubbling; make sure to check and stir regularly. When everything is really soft, take off the heat and let cool.
Once cool you can blitz it to desired consistency. Reheat to serve, (you may wish to add a little cream to make it creamier) ladle into warmed bowls and serve.
You could substitute a medium size potato for the slice of bread, just peal, cut up small and add before the stock. Make sure the potato is soft before adding the spinach. This may add time to your cooking.