In a large soup pot, bring stock to the boil. Break up the cauliflower and broccoli florets and add to the pot along with the leek and garlic. Bring back to the boil then reduce heat and simmer till all the veg is soft.
Blitz the soup with a hand blender till smooth. Simmer for a further 15 to 20 minutes. Add salt and pepper to taste and serve.
Lovely served with crusty bread topped with a little melted cheese.