Preheat the oven to 400F.

To make the onion ash, cut the onion in half through the root end. Remove the skins and the top and bottom of the onion. Peel each layer of the onion and place them into a frying pan big enough to spread the onion in a single layer. Place the pan in the oven and cook for about an hour or until all of the onion pieces are completely burnt. Remove the onions from the oven and let cool. If there are pieces that are at still moist, cook them further or set aside to use in another dish.

Place the burnt onions into a food processor or grinder and process until there are no chunky bits left. Reserve.

Cut the top and bottom off the celeriac. Using a peeler or your knife, remove the skin.

Preheat oven to 375F.

In a small pan over medium heat, add a small amount of oil and place the celeriac flat side down. Cook for a few minutes until the celeriac has caramelized. Turn the celeriac over to the other flat side and place into the oven. Cook for approximately one hour or until a knife slides easily through the centre. Remove the pan from the oven and let the celeriac cool to room temperature.

Once the celeriac is cool enough to handle, brush the Dijon mustard onto the sides. Sprinkle with salt. Using the onion ash, coat the sides of the celeriac to create a thin layer. The whole celeriac should now be black except for the top and bottom. Place the celeriac back into the oven for a few minutes to warm it through. Remove and set aside.

While the celeriac is warming, add the honey to a small pot and cook it over medium heat until it turns to a dark amber colour. Remove the pot from the heat and stir in the vinegar. Add the chilis.

To serve, cut the celeriac into round slices, season with a pinch of salt then drizzle with the spicy burnt honey gastrique and herbs.