Place the sweet potato, beetroot and onion on a baking tray. Drizzle with the olive oil and season with salt and pepper. Roast in the oven for 40–50 minutes, or until the vegetables are just soft. Place the vegetables in a baking dish.
In a large bowl, rub the flour and butter together until it resembles fine breadcrumbs. Stir in the nuts and season with salt and pepper.
Sprinkle the crumble over the top of the vegetables and place in the oven for 20–30 minutes, or until the crumble is golden.
Put four of the largest oysters aside in a bowl with 25ml/1fl oz of white wine.
Heat 1 tablespoon of the butter in a frying pan and sweat the shallot for 5 minutes. Add the remaining wine, stock, oyster juice and the remaining oysters. Simmer for 5 minutes. Stir in the crème fraîche then transfer to a food processor or blender and blitz.
Season with salt, pepper, nutmeg and lemon juice, to taste. Place the soup in a saucepan and add the marinated oysters. Simmer the oysters in the soup for 2 minutes.
Meanwhile, for the salmon, heat the oil and butter in a grill pan. Sear the salmon on both sides until it just turns pink.
Place the salmon pieces in the centre of four plates. Pour over the oyster sauce and garnish with the sea vegetables. Serve with the vegetable crumble alongside.