Preheat the oven to 180°C/fan 160°C/gas mark 4. Add the lime zest to a small baking tray and bake for 3-4 minutes until dry and crisp. Meanwhile, add the cumin seeds to a small frying pan over a medium heat and toast until aromatic, being careful not to burn the seeds. Add the lime zest and cumin seeds to a pestle and mortar and grind down to a fine powder.
Put all the remaining ingredients in a blender. Add the ground lime zest and cumin along with 150ml water and blend to a smooth paste. Add the paste to a large jug, making sure to scrape the blender clean.
Place a fine mesh sieve over a large bowl and line it with muslin or a clean tea towel. Gradually add the paste, letting it drip through. As it goes through, add another 600ml water bit by bit until all the mixture has gone through the sieve.
Carefully scoop up the muslin/tea towel and gently squeeze out any excess liquid into the bowl. Discard the residue and add water if necessary to make it up to 700ml. Taste the lemonade and add more salt, sugar or lemon juice according to your taste. Divide amongst 4 tumblers filled with ice to serve.