Start by cutting off the rough outer surface of the celeriac and then cube it in 1" cubes.
Mix the soy sauce and Ketjap manis in a plastic bag and add the celeriac cubes. Tie up the bag and shake it to mix the ingredients. Keep in the fridge for 1-2 hours.
Aioli
Add wild garlic, egg yolks, vinegar in a food processor or tall jar and blend to a smooth puré. Now add the oil in a thin stream, blending continously. Add salt to taste.
Grill
Fire up your grill, prepare it for high indirect heat, it should be running at 200° C / 400° F. Once it is up to temp it's time to place the celeriac cubes evenly on the indirect side. Grill them with the lid closed for 10-15 minutes. How long depends on what texture you prefer. Poke them with a fork (or similar) and see.
If you feel they need more char on the surface then grill them for 1-2 minutes over high direct heat before bringing them in to serve, with the wild garlic aioli in a small dipping bowl.