Allow the orange and onion to cool enough to handle. Working over a bowl to catch the juice, remove the flesh of the orange from the peel using a paring knife. Also remove the pith and seeds from the orange, and squeeze the juice from any flesh that is left on the peel. Next add the agave, cilantro and coriander, stir gently. Slice the grilled onion into small pieces and stir into the orange mixture. Add salt and pepper to taste, I like this relish to be sweet and salty and have plenty of pepper, so go crazy.