Preheat the oven to fan 180C/conventional 200C/gas 6. Pour the juice of one lemon into a large bowl. Slice your celeriac into pieces about the thickness of a £1 coin, tossing them into the lemon juice as you go, then add the potato slices.
In a large ovenproof dish, layer the celeriac and potato with the slices of salmon, sprinkling with your small handful of chopped dill, 1 chopped onion and drizzling a little cream over each layer. Season with a little salt and lots of pepper at each stage. You should have three layers of potatoes and celeriac sandwiching two of salmon, onion and dill. Finish with the remaining cream.
Cover the dish with foil and place on a tray. Bake for 45 minutes, then remove the foil and bake for another 30-40 minutes until bubbling and the top is crispy and golden. Test that the vegetables are cooked by inserting a skewer or small knife – it should push in easily. If you like, you can flash it under a hot grill to get the top layer really crispy. Leave to cool slightly then serve straight from the dish with a crisp green salad and some chilled white wine.