Preheat the oven to 180°C.
Melt half the butter in a deep ovenproof pan and fry onion until soft.
Remove onions and fry a layer of potato slices in the same pan until golden.
Layer the fried onions and remaining potatoes alternately in the pan.
Add a layer of 100g smoked salmon before the final layer of potatoes.
Pour the remaining melted butter over the assembled layers.
Bake in the oven for 40 to 45 minutes until cooked through.
Preheat the grill to its highest setting.
Top with grated cheese and grill until golden-brown.
Whisk yoghurt, lemon juice, zest, and dill to make the dressing.
Serve wedges topped with remaining smoked salmon and the dressing.