Preheat the oven to 200°C, fan 180°C, gas mark 6. Bring a small pan of water to the boil. Add the eggs and boil for 7 minutes. Drain, then set aside to cool before peeling and cutting into quarters.
Bring a large saucepan of water to the boil, add the spinach and blanch for 2-3 minutes. Drain, squeeze out the excess water and set aside.
Slice the unpeeled potatoes into thin rounds — this is easiest to do using a mandoline slicer. Set aside.
In a large pan, melt the butter and add the onion. Cook for 5 minutes before adding the garlic for the last minute. Stir in the flour and cook for 1-2 minutes.
Remove from the heat and slowly whisk in the milk. When all the milk has been added, return the pan to the heat. Simmer for 5 minutes, stirring, until thickened.
Add the hardboiled eggs, salmon, capers, spinach and dill and transfer to a 1.5 litre pie dish. Arrange the potato slices on top and brush them with the oil. Bake for 30-40 minutes, until the potatoes are crisp and golden.