Take a baking dish or roasting tray large enough to fit the salmon and line it with cling film. Trim the sides of the salmon if necessary to neaten the edges, then place in the dish, skin side down.
To make the cure, break up the coriander seeds using a mortar and pestle. Place in a bowl with the beetroot, orange zest, fine sea salt, sugar and a couple of twists of black pepper and mix well.
Spread the cure mixture over the salmon, making sure the flesh is completely covered. Cover with a sheet of cling film and sit another dish or tray directly on top of that. Weigh this down with some tins or a heavy pan and leave in the fridge for 8 hours or overnight.
Meanwhile, make the salad. Put the lemon juice into a large bowl. Slice the celeriac and apple into matchsticks, and toss immediately in the lemon juice to prevent them from going brown.
In a separate bowl, mix together the sour cream, mayonnaise and parsley. Season with salt and pepper, then add to the celeriac and mix well. Taste and adjust the seasoning as necessary.
Once ready to serve, remove the salmon from the cure, wiping off any excess. Place the fish on a chopping board, skin side down, and thinly slice the flesh at an angle into pieces about 1–2mm thick, flicking your knife when you reach the skin to slice the flesh away from it. Transfer the salmon pieces to a plate.
Garnish the apple and celeriac salad with the poppy seeds and serve alongside the sliced salmon with some toasted rye bread.