Preheat the oven to 400°F. Cut the cauliflower into small florets and spread them out on a baking sheet lined with parchment paper. Add the halved onions and whole garlic cloves, drizzle with olive oil, and roast everything in the oven until the cauliflower is golden brown and tender, tossing halfway through.

Meanwhile, bring water to a boil in a large pot and add the crumbled chicken stock cube. Remove the roasted onions from their skins and squeeze the soft flesh out of the garlic cloves.

Set aside a few nice pieces of cauliflower for garnish and add the rest to the pot along with the onions and garlic. Blend the soup until smooth with an immersion blender, adding a little extra water if needed to reach the desired consistency. Season with pepper and salt.

Reheat the soup if necessary and ladle into bowls. Garnish with the reserved cauliflower and the gray shrimp, and serve immediately.