Sweat the onion in the butter for about 5 minutes until it is soft and translucent; add the celeriac and coat with the buttery onion. Cook off for 2-3 minutes and pour in the vegetable stock and toss in the kelp. Bring to the boil and simmer for about 20 minutes until the celeriac is soft.
Remove the kelp from the soup and add the cream, truffle oil, nutmeg and some seasoning and blend the soup until its smooth. Keep soup hot until ready to serve.
To make the duxelle, fry the shallots and garlic in the butter for a couple of minutes until they are soft. Stir in the Portobello mushrooms and cook until you have a rough pâté consistency and season to taste.
Make a quenelle or oval shape with the duxelle using two teaspoons and place the quenelle at the bottom of a warm soup bowl. Arrange 3-4 truffle shavings around it and garnish with some micro cress.
To serve, pour the soup into a warm jug and place the jug and soup bowl on a plate with a slice of walnut bread.