Preheat your oven to 200ºC. Brush the butternut squash all over using 1 tbsp of the olive oil and place into the oven on a baking tray. Roast for an hour until soft.
Meanwhile, heat the remaining tablespoon of olive oil in a large pot over a medium-low heat. Add the shallot, garlic and ginger and cook, stirring often, until the shallot is translucent. Add the chopped celery, lemongrass, star anise, cinnamon stick and clove to the pot. Continue to cook for another 5 minutes to slightly soften the celery. Crumble the stock cube into the pot and pour in the water. Turn up the heat and bring the liquid to the boil then turn down and leave to simmer while the squash roasts.
Once the squash is roasted and completely tender, let it cool slightly then cut off the top and base and cut in half lengthways. Scoop out the seeds and discard them. Cut or peel the skin away from the flesh.
Remove the lemongrass, star anise, cinnamon and clove from the pot and discard them. Add the flesh of the squash and the lemon/lime juice to the pot and simmer for 5 more minutes. Blend the soup until smooth using a hand blender right in the pot or pour the soup into a free-standing blender and carefully blend.
For the topping, stir together 3 heaped tsp full-fat yogurt, 1 tsp tahini, zest of an orange, juice of ½ an orange, 1 clove of garlic (crushed) and a pinch of salt, loosening it with a tablespoon of water if desired.
Divide the soup between a few bowls and serve with a few spoonfuls of topping and any desired garnishes.