Cut the beet into 3/4-inch/2 cm cubes and toss with the oil in a bowl.
Gently toast the coriander and cumin seeds in a dry frying pan to release their aromas, 30 to 60 seconds.
Tilt into a mortal and pestle and grind the seeds to crush slightly and release their flavour (not to turn them into powder, however).
Add the remaining spices, and toss with the beet to coat evenly.
Cover and leave to marinate several hours.
Before serving, toss with the fresh coriander.
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