1. In a large saucepan or stock pot melt the butter, add the onion and cook slowly until translucent, stirring constantly. Add the garlic and cook for a further 2 minutes taking care not to burn. Stir in the mustard powder.
2. Meanwhile, remove the leaves from the cauliflower and cut into florets and steam with the broccoli until almost tender. Add to the onion, and cover with vegetable stock. Add the potato and bring to the boil, then simmer for 15 minutes or until the potato is cooked.
3. Purée the soup in a food processor, return to the pan, adding the mustard and crème fraiche. Bring back to the boil, reduce the heat and simmer for 5 minutes. Season to taste with sea salt and black pepper.
4. To serve, place 50g/ 2 oz of grated cheese in the centre of a warmed soup bowl. Surround with the soup and top with croutons.
Note: Prep Time: 10 minutes, Cook Time: 25 minutes, Total Time: 35 minutes