In a wide saucepan over a low heat sweat the diced onion in the olive oil. It must cook slowly and not brown, approximately 10–15 minutes.
While this is cooking carefully lift out the ceps from their water – use your fingers or a spider. This ensures that the inevitable grit remains behind in the water. Pat dry and dice, then add to the onions and continue to sweat together for 10 minutes further. During this time carefully strain the cep soaking water through a coffee filter or fine tea strainer. When the cep and onion mix has had the 10 minutes, add the cep liquor and turn the heat up to medium. Cook until the cep water is nearly completely evaporated. Set aside to cool.
Clean and sterilise a
Grate the truffles (straight from the freezer if frozen) through the same aperture of the grater as you would Parmesan, directly over the cooked cep and onion mix. Stir, taste and season, then add half the truffle oil. Using a rubber spatula, transfer this mixture to the jar. Pat down in the jar, wiping the edges inside the glass with a clean cloth. Top up the jar with the remaining truffle oil: the layer of truffle oil should cover the relish completely. Keep in the fridge; if unopened, almost indefinitely.
To use the relish, stir some more truffle oil into it.