Peel and finely chop the onion and sweet potatoes. Warm a large pan over a medium heat for 1 min, then add 1 tbsp oil with the onion and potatoes. Season with a pinch of salt and pepper. Put a lid on the pan. Turn the heat down slow. Sweat for 10 mins, stirring now and then, till the veg start to soften.
While the veg sweat, peel and crush the garlic. Finely slice the coriander stalks (keep the leaves for later). Finely grate the zest off the lime. Slice the chipotle and discard the seeds (unless you like a really spicy soup).
Crumble the stock cube into a jug. Pour in 750ml boiling water and stir to dissolve the stock cube.
Stir in the garlic, coriander stalk, lime zest and chipotle. Add 1 tsp ground cumin. Pour in the stock. Cover, turn the heat up and bring to the boil, then turn the heat down and simmer for 15 mins to completely soften the veg.
While the soup simmers, pull the silks and threads off the corn cob. Slice a slither off the base so it stands upright on your board. Run a sharp knife down it to slice off the corn kernels.
Warm a dry frying pan for 1 min. Add 1/2 tbsp oil and the corn kerns. Stir and fry for 4-5 mins till they are browned. Take off the heat and set aside.
Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Juice the lime and stir it into the soup. Taste and add more salt and pepper if you think it need sit. Ladle into warm bowls. Top with the sweetcorn and coriander leaves.