For the salted cucumber and horseradish, place the cucumber slices into a colander and sprinkle with plenty of salt. Mix well and leave the contents to drain over the sink for half an hour.
Rinse the salt off the cucumber with cold water, then leave to drain. Gently wring out any excess moisture from the cucumber with your hands, then set aside.
In a clean bowl, mix the horseradish with the crème fraîche and mustard powder, making sure the mustard powder is well combined with no lumps. Set aside.
Season the mackerel fillets on their skin side with salt and freshly ground black pepper.
Heat the butter in a frying pan until it is foaming, then add the fillets skin-side down. Place a heat-proof plate onto the cooking fillets, as this will make sure they stay flat and cook evenly. Cook for 4-5 minutes, or until nearly cooked through, then turn the fillets and cook for 30 seconds, or until just cooked through.
To serve, place a small handful of the cucumber onto the toast. Place the cooked mackerel fillets onto the cucumber. Place a dollop of the horseradish sauce on top and garnish with a little of the sliced red onion and a squeeze of lemon.