Divide cauliflower into small florets, peel and dice potatoes. Puree garlic and ginger with some water.
Heat oil in a pan, add cauliflower, potatoes and water and bring to the boil. Simmer covered 10 minutes.
Add garlic-ginger paste, turmeric, mustard seeds, coriander and cumin seeds and boil stirring for 3 minutes. Douse with the soup and season to taste with salt and pepper.
Bring to a boil and simmer covered for 20 minutes. Remove the pot from the heat, let cool and puree the soup. Fold in Yogurt.
Heat soup gently.