Place potatoes in a large pot and cover with a couple inches of water. Bring to a boil and cook potatoes until fork tender, about 10 minutes. Drain potatoes and return to pot.
Add onion, yogurt, lemon zest, salt and black pepper. Mash until well combined. Let potatoes sit at until cool enough to handle.
While potatoes are cooling, mix ingredients for sauce together and season with salt and black pepper.
Place beaten egg in one bowl and place panko in another. Heat olive oil in a large skillet on medium-high heat. Scoop golf ball sized scoops of mashed potato, roll into a ball, dip in beaten egg then the panko. Place in the hot skillet and flatten slightly with a spatula. Fry 3 minutes per side until golden, flip and fry 2 minutes on the other side.
Remove potato cakes to a serving plate. Top with slices of smoked salmon. Dollop with mustard-dill sauce.