Toss together 1 tablespoons of olive oil with cauliflower, turmeric and coriander. Transfer to a rimmed baking sheet or baking dish, and roast in preheated oven for 20 minutes, until golden brown.
Meanwhile, heat remaining olive oil in a soup pot over medium heat. Add onions and sauté until translucent, about 7 to 10 minutes. Add in garlic, salt, and pepper.
Once cauliflower is roasted, add it to the soup pot with the onions and garlic. Pour the broth into the baking dish and use a spatula to scrape up any bits of cauliflower or spices left on the dish. Pour into the soup pot. Stir well, cover and bring to a boil. Reduce heat to a simmer and simmer for 15 minutes, stirring occasionally.
Remove soup from heat and use an immersion blender to blend the soup until smooth. Or use a blender, but be careful!