1. Steam 2-3 potatoes, peel, and slice 0.5cm thick.
2. Slice half an onion thin and sauté in 60g butter. Set the onion and butter aside separately.
3. Beat 1 egg into 300ml milk, then add salt and pepper and mix.
4. In an oven-safe dish, layer potato, smoked salmon, onion, and the sauce from step 3; repeat once more with potato, smoked salmon, onion, the remaining sauce, and the reserved butter from step 2. Bake at 180°C for 30 minutes.
5. Top with chopped scallion.