Add 2 tsp coriander seeds to a pan. Toast them over a medium heat, shaking the pan, for 2-3 mins till the seeds smell aromatic. Tip into a bowl and set aside.
Prepare the veg: peel and finely chop the onion. Peel and finely chop the carrots. Peel and grate the ginger. Warm the pan you toasted the coriander seeds in for 1 min, then add 1 tbsp oil and the onion, carrots and ginger. Season. Cover and sweat over a low heat for 10 mins till the veg start to soften.
Cut the clementines in half (no need to peel). Slice 2 thin slices off one half and set them aside for garnishing later on. Juice the clementines, keeping the juiced fruit halves. Finely chop the chilli. Add the juice and juiced halves to the veg. Crumble in the stock cube. Add the toasted coriander seeds and most of the chilli. Pour in 700ml boiling water. Cover and bring to the boil. Turn the heat down and simmer for 15 mins.
Fish out the clementine halves. Blitz the soup in a blender, or use a hand-held blender. Taste and season if it needs it. Serve in warm bowls garnished with the clementine slices and the remaining chilli.