Coarsely grate the celeriac into a large bowl and set aside. Coarsely grate the potatoes into clean tea towel, draw up the corners, twist tightly and squeeze out the excess liquid. You want the potato as dry as possible. Add the potato and the egg to the celeriac and mix well to combine. Season with salt and pepper.
Generously cover the base of a large frying pan with oil and warm over a medium heat. Place a quarter of the rösti mix into the hot oil, use a fork to gently shape into a round, but don’t squash flat. Cook the rösti for five minutes or until the base is crisp and golden. Carefully flip over, and cook the other side. Remove, drain on kitchen paper, and keep warm in a low oven and cook the remaining 3 röstis, one at a time.
To serve, divide röstis onto on four plates, top each with a slice of Huon smoked salmon, a dollop of crème fraiche and sprigs of dill. Serve with an optional wedge of lemon on the side.