Make the potato cakes. Bring a large pan of salted water to the boil. Tip in the potatoes and cook for 10-15 mins until soft. Drain well, then return to the hot pan for 1-2 mins to allow any excess moisture to evaporate. Pass the potatoes through a ricer or sieve to get a really fine mash.
Weigh out 250g of the mash and place in a bowl (you shouldn’t have much left over). Tip in the flour, baking powder and 1 tsp salt. Mix together until the mixture forms a soft dough. Shape into a ball, then tip out onto a lightly floured surface. Roll out into a circle about 20cm across and 1cm thick. Cut into 8 triangular wedges and generously dust all over with flour. The potato cakes can be made up to this point 1 day ahead and chilled or frozen for up to 1 month.
Set a large, non-stick frying pan over a medium heat (no need to add oil). When hot, dry-fry the cakes for about 3-5 mins on each side until golden and cooked through. You may have to do this in batches. Stir a little water into the soft cheese until it is spoonable. Place the potato cakes on plates, top with dollops of soft cheese and a smoked salmon slice, then scatter over the red onion, capers and lemon zest to finish.