Place the carrots on the baking sheet lined with baking paper.
Add 2 tablespoons oil and ½ teaspoon of the salt. Toss until the carrots are lightly coated in oil and seasonings.
Roast the carrots for about 25 minutes. Set aside.
In the meantime, over medium heat, place your large pot and add 1 tbsp of oil , onions and garlic, and stir occasionally. Cook for about 5 minutes.
Add the coriander and cumin and keep stirring for about 1 minute. Pour in the vegetable broth and water, and scrap up any browned bits on the bottom with a wooden spoon.
Add the roasted carrots to the pot and bring to he boil. Then reduce the heat and simmer for 15 minutes.
Remove the pot from the heat and let it cool for a few minutes. Carefully blend, I recommend using a stick blender, otherwise transfer to a blender (it will be still hot so be careful) and blend in batches if necessary.
Finally, add butter, lemon juice, and black pepper to taste. Blend until completely smooth. Taste and adjust seasoning.
Keep this soup for up to 4 days in the fridge, covered. Or freeze it for several months.