Put the carrots and onion into a roasting tin. Add the olive oil and a big pinch of salt, and season with a good grinding of black pepper. Add the ground spices, mixing well to coat.
Cover the roasting tin tightly with foil and roast the carrots and onions for about 35 minutes until tender.
Meanwhile, rinse the lentils well in cold running water and put in a saucepan with a litre of water. Place over a moderate heat and bring to a boil. Skim off any froth that rises to the surface then reduce the heat and simmer for 15-20 minutes until cooked and very soft.
Blend the lentils until smooth, then add the roasted vegetables and process until your preferred texture. Check the seasoning and add more water if you want a looser soup.
To serve, slick each bowl of soup with a little more olive oil and sprinkle over the zaatar.