Place the potatoes and onion in a clean tea towel and squeeze to remove excess moisture. Season well. Heat the butter and 1 tablespoon of the oil in a nonstick frying pan.
Tip in the potato mixture and spread out to make an even layer, then cook for about 10 minutes until golden. Invert the rosti on to a plate, then carefully slide back into the pan the other way up to cook the other side. Cook for a further 5–8 minutes until cooked through and golden all over.
Meanwhile, mix the lemon juice with the remaining oil and toss with the rocket. Cut the rosti into wedges and serve with slices of smoked salmon, the rocket salad and lemon wedges.