Preheat your oven to 190°C (170°C with a fan).
Line a baking tray with parchment paper.
Spread the cauliflower and onion in a single layer on the tray.
Spread the cauliflower and onion in a single layer on the tray.
Lightly spray with olive oil and roast for 15 minutes.
Add the garlic cloves and continue roasting for another 15 minutes, or until the vegetables are lightly golden and tender.
Set aside 1/2 cup of the roasted cauliflower.
Transfer the remaining cauliflower, onion, and squeezed garlic (discarding the skins) to a large saucepan.
Add the vegetable stock, white beans, and a sprig of rosemary.
Bring to a simmer over medium heat, cooking for about 10 minutes or until the cauliflower is very tender.
Remove the rosemary sprig and take the pan off the heat. Use an immersion blender to puree the soup until smooth.
Ladle the soup into bowls and top each serving with the reserved roasted cauliflower.
Drizzle with chili oil and sprinkle with chopped pistachios before serving.