Preheat the oven to 180ºC. Line a large baking tray with non-stick parchment paper, and brush it lightly with olive oil.
In a mortar and pestle, finely grind the cumin and coriander seeds. Mix the ground spices with the paprika.
Put the cauliflower florets in the prepared baking tray. Spread the spices and salt evenly on top of the cauliflower. Roast the cauliflower in the oven for 30 to 35 minutes, or until golden brown. When the cauliflower is done roasting, heat 1 tablespoon of the olive oil in a large pan, over medium heat. Add the onion and fry for 5 to8 minutes, or until translucent.
Add the stock to the pan and bring to a boil. Finally, add the roasted cauliflower and turn off the heat.
Now, using an upright blender, purée the soup until very smooth. Add the ginger juice, mix well to incorporate, and serve immediately.