Place the smoked mackerel, cream cheese, creamed horseradish and lemon juice in a food processor. Blend into a well-mixed paste.
To make the rösti, bring a large pan of water to the boil. Add in three of the potatoes and boil them in their jackets for 8 minutes exactly; drain and allow to cool.
Once the part-boiled potatoes are cool enough to handle, peel them and coarsely grate them into a bowl. Peel the remaining uncooked potato and grate into the bowl too, together with the apple.
Mix in the garlic, season well with salt and freshly ground pepper and shape into 10cm rounds, making 8 rösti in all.
Heat the vegetable oil in a large, heavy-based frying pan. Add in the potato rosti and fry, in batches, over a medium heat until crisp and golden on both sides. Drain on kitchen paper.
Dress the watercress with olive oil and balsamic vinegar.
To serve, place a rösti each on four serving plates, top with watercress, then another rösti, then finish each serving with a portion of smoked mackerel pâté and a lemon wedge.
