Lightly spray the base of a large saucepan with cooking oil, then place over a medium heat and add the onion and garlic. Add 1 tablespoon of water then cover and cook, stirring occasionally over a low heat for 5 minutes. Add the ground cumin and ground coriander and cook for a further 2 minutes.
Add potatoes and broccoli to the pan, stir well and add the stock cubes and 1L of water. Slowly bring to the boil, reduce the heat, cover and simmer for around 20 minutes until the vegetables are tender. Remove from the heat and allow to cool slightly.
Blend the soup until smooth using a hand liquidiser or in a food processor. Return to the pan and stir in the milk. Slowly reheat without boiling. Stir in the fresh coriander and season to taste. Serve in warmed bowls.