Step 1 Preheat oven to 180°C. Line a large baking tray with baking paper. Put pumpkin on prepared tray and spray with olive oil; roast for 30 minutes, or until tender and golden. Put one-third of the roast pumpkin in an airtight container and place in fridge (reserving for Friday’s Roast pumpkin, leek and mushroom cannelloni recipe, see Tip).
Step 2 Meanwhile, set a large saucepan over medium heat and spray with olive oil. Add leek and celery to pan; cook, stirring occasionally, for 5 minutes, or until soft. Add garlic to pan and cook, stirring, for 1 more minute.
Step 3 Add potatoes and vegetable stock to pan; cover and simmer for 12 minutes, or until potatoes are tender. Add remaining roast pumpkin to pan and simmer for 3 more minutes, then leave soup mixture to cool slightly.
Step 4 Blend soup with a stick blender until smooth; warm over medium heat. Meanwhile, preheat grill to high. Scatter sourdough with parmesan (if using) and grill until toast is golden and parmesan melts; scatter with hazelnuts.
Step 5 Divide soup among serving bowls, add a dollop of yoghurt to each and serve with toasts.