Peel the sweet potato. Chop it into 2cm dice. Set a pan over a medium heat for 2 mins. Add 1 tbsp oil. Add the sweet potato dice. Season well. Sizzle over medium heat till just golden, about 7 mins.
Peel the onion. Halve and deseed the pepper. Finely dice the onion, pepper and celery. Stir the diced veg in with the sweet potato. Cook for 5 mins, stirring occasionally, till the veg are tender.
Peel and finely chop the garlic. Add to the pan, along with ½ tsp Cajun spice and the zest from the lemon.
In the pan, mash 2-3 cubes of sweet potato a bit, to give you a sort of paste. Stir it round the pan to mop up all the spices. Let it cook down a little, being careful not to let it burn. Add a squeeze of lemon juice to loosen everything up.
Pour in the tomatoes. Fill the tin with water (around 400ml) and add to the pan. Simmer for 10 mins or till the potato is cooked through. Taste and add more salt, pepper or cajun spice if it needs it.
While the soup simmers, slice the spring onions into 1-2cm chunks. Get a small frying pan smoking hot. Rinse the spring onions. Toss into the pan with a little water clinging to them. Add a little salt. Sizzle (no oil needed) till lightly charred.
Ladle the soup into bowls and top with the charred spring onions to serve.