Peel your sweet potato. Cut into a 2cm dice. Get a large frying pan or pot hot. Add a gloss of oil. Add the potato. Season well. Sizzle over medium heat till just golden, about 7 mins.
Finely dice your onion, celery and pepper. Swirl it in with the sweet potato.
Peel and finely chop your garlic. Add it to the pan, along with the Cajun spices and the zest from your lemon.
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, careful not to burn. Add a squeeze of lemon juice to loosen everything up.
Pour in your tomatoes. Fill the tin with water. Add it. Simmer for 10 mins or till the potato is cooked through. Taste. Add more spices to your liking.
Slice your spring onions into 1-2 cm chunks. Get a small frying pan smoking hot. Rinse your spring onions. Toss into the pan with a little water clinging to them. Add a little salt. Sizzle (no oil needed) till lightly charred.
Finely chop your parsley. Stir half of it into the soup. Dish out. Garnish with spring onions, a gloss of oil and remaining parsley.