Spray a pan with non-fat cooking spray.

Add the diced onions and celery to the pan.

Cover the pan and cook on low heat until the onions and celery are tender and almost lose their color, stirring occasionally.

Blend in the cornflour.

Add the potatoes and chicken or vegetable stock.

Stir constantly until the mixture boils.

Reduce heat and simmer for 30 minutes.

Add the frozen spinach and stir until it is defrosted and heated through in the soup.

Add the skim milk and heat through.

Serve hot.