Preheat your oven to 180°C (fan oven), 200°C (conventional) or gas 6. Grate the zest off the lemon (optional), squeeze the lemon, and put (both) into a large bowl. Slice the celeriac in a food processor into pieces about the thickness of a coin, tossing them around immediately in the lemon juice, then add the sweet potato slices.
In a large greased ovenproof dish, layer the celeriac and sweet potato slices with the pieces of salmon, sprinkle with a small handful of chopped dill. Then distribute the onion slices and parmesan, and drizzle a little cream over each layer. Season with a little salt and lots of pepper at each stage. You will end up with three layers of sweet potatoes and celeriac, and two layers of salmon, onion and dill in between. Pour the remaining cream over the vegetables, and sprinkle the rest of the parmesan over the top.
Cover the dish with tin foil and bake for 45 minutes, then remove the foil and bake for another 30 to 40 minutes until the cream is bubbling and the top is crispy and golden. Insert a skewer or a thin-bladed knife into the ovenbake. If you can push it in easily, the vegetables are cooked. Let it sit outside the oven for a few minutes, as it will be boiling hot, then serve with a leafy green salad.