Dry-fry the coriander seeds in a small, heavy-based frying pan over a medium heat for 2–3 minutes until they give off a nutty aroma, then crush them in a mortar with a pestle to release the flavour.
Peel the bitter orange, remove the pith, and finely slice the rind, discarding the rest.
Place the sliced olives in a bowl. Add the sliced orange rind, crushed coriander seeds, olive oil and lemon juice and mix well. Leave to stand for at least 10 minutes to allow the flavours to mingle. Serve as a starter with bread, if liked, or as a side dish to accompany a tagine.