Remove marron meat from shell and place on a skewer. Heat over gentle coals or on a BBQ for 4-5 minutes, turning once, until cooked.
For the kelp oil, pour vegetable oil into a medium-sized pan. Then place kelp in the oil and cover. Cook on a low heat for 1 hour.
In a separate pan, warm the milk and diced onion for 30 minutes. Remove from heat and add a shot of coffee. Cover and leave for a further 30 minutes. Strain through some fine muslin cloth and season with salt.
Wrap Jerusalem artichokes in foil and bake in oven at 190°C for 60 minutes, until soft. Once cooked, slice open and remove the white meat inside. Season with salt.
To plate, place the marron in a bowl and arrange pieces of Jerusalem artichoke around it. Cover the dish with warm coffee milk, followed by kelp and one tablespoon of kelp oil.