Peel and finely chop the onion. Trim the carrots, scrub them and finely chop. Warm a pan for 1 min, then add 1 tbsp oil and the chopped veg. Season with salt and pepper. Turn the heat down very low and put on the lid. Gently sweat the veg for 15 mins till they’ve softened.
While the veg sweat, zest the orange and juice it. Crush 4 cardamom pods with a pestle or the end of a rolling pin. Discard the papery skins but keep the seeds.
Stir most of the orange zest into the veg with the cardamom seeds. Add most of the thyme sprigs. Crumble in the stock cube, then pour in 800ml boiling water (or use hot homemade stock). Cover. Turn up the heat and bring to the boil. Turn the heat down and simmer for 10 mins till the veg are soft.
Fish out the thyme sprigs. Ladle the soup into a blender and blitz, or use a hand-held blender in the pan. When the soup is smooth, swirl in half the blood orange juice. Taste and add more salt, pepper or orange juice if it needs it. Ladle into warm bowls and top with the remaining orange zest and thyme leaves to serve.