Put a medium pan over a medium-high heat. Add 1 tbsp cumin seeds. Toast for 2-3 mins, shaking the pan, till the seeds smell nutty and start to pop. Tip them out into a bowl.
Peel and finely chop the onion. Trim, peel and finely chop the carrots. Put the pan back on a very low heat and add 1 tbsp oil. Add the onion and carrots. Season with a good pinch of salt and pepper. Cover and cook for 10 mins, stirring occasionally, till the veg start to soften.
Peel and crush the garlic. Finely grate the zest from the lemon and juice it. Stir the garlic, half the cumin and half the lemon zest into the veg. Pour in 750ml boiling water (or homemade stock). Bring to the boil. Turn the heat down a little. Simmer for 10 mins till the veg are soft.
Swirl the lemon juice into the soup. Blitz it in a blender till smooth (or use a hand-held blender). Taste and add more salt and pepper if you think it needs it.
Ladle the soup into warm bowls. Top with the rest of the lemon zest, the parsley leaves and the remaining cumin seeds.