Peel the potatoes and boil in salted water until fork-tender. Let cool, then slice.
Thinly slice the onion and cook in the butter until soft, reserving the melted butter.
Whisk together the cream, milk, and eggs; season with salt and pepper.
Layer the sliced potatoes, smoked salmon, cooked onions, and dill in a baking dish, pouring a bit of the cream mixture between each layer. Finish with a final layer of the cream mixture on top.
Bake at 175C for about 30 minutes, until the top is golden and bubbly.
Drizzle with the reserved melted butter and garnish with fresh dill.
