Melt the butter and olive oil in a large, heavy-based saucepan over a medium heat, then add the potato, onion and most of the cauliflower florets. Reserve the remaining florets for a garnish. Sauté the vegetables for 4-5 minutes, then cover with a lid and reduce the heat a little, allowing the steam to soften them.
Add the stock and bring the mixture to the boil. Stir in the milk then bring the mixture back to the boil. Simmer for 15 minutes until soft, then remove from the heat.
Puree the soup in batches in a food processor. Transfer back to the saucepan and adjust the seasoning.
Lightly crush the toasted hazelnuts, then ladle the hot soup into serving bowls. Scatter small handfuls of the hazelnuts into the centre of the soup, then sprinkle pinches of the turmeric all over.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.