Warm a dry pan over a medium heat. Add 1 tsp each cumin and coriander seeds. Toast, shaking the pan occasionally, for xx mins till they smell aromatic and are popping. Tip into a mortar and grind with a pestle, or tip into a bowl and use the bottom of a jam jar to grind them.
Peel and grate the garlic and ginger. Halve the chilli. Scoop out the seeds and white bits (or leave them in for extra heat). Finely chop them. Finely chop the coriander stalks - keep the leaves for later. Finely grate the zest from the lime.
Put the pan back on the heat for 1 min. Add 1 tbsp olive oil and the garlic, ginger, chilli, coriander stalks and lime zest. Add the ground spices. Cook and stir for 1 min. Squeeze in the tamarind paste. Tip in the tomatoes. Add 600ml boiling water.
Season the soup with salt and pepper. Cover. Bring to the boil, then turn the heat down and simmer for 10 mins to mingle all the flavours. Ladle the soup into blender and blitz till smooth, or use a hand-held blender in the pan. Squeeze in a little lime juice. Taste and add more salt, pepper or lime juice if you think it needs it. Serve in warm bowls, topped with the coriander leaves.