Scoop out about half of the pulp from each lemon quarter and discard. Coarsely chop the lemon quarters, then put them in a food processor or blender. Add the shallot, parsley, and olives and pulse several times until minced but not puréed. Transfer to a bowl and stir in the coriander seeds and vinegar. Season with the salt and pepper. For a chunkier relish, chop the olives coarsley by hand and stir them into the minced lemon mixture with the coriander and vinegar.
Serve at once, or cover and refrigerate for up to 3 days. Bring to room temperature before serving.