Preheat your oven to 220°C/200°C (fan)/425°F/Gas 7.
Bake the salmon fillet for 15–20 minutes, or until it’s cooked through. Remove from the oven, let it cool slightly, and flake into chunks. Lower the oven temperature to 190°C/375°F/Gas 5.
Place the sliced potatoes in a pot of boiling water and simmer for 4–5 minutes. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the sliced onion and cook for 4–5 minutes until softened. Toss in the spinach and cook until wilted, about 2 minutes. Add the flaked salmon and stir to combine, then set aside.
To make the cheese sauce, combine milk, butter, and flour in a saucepan. Heat gently, whisking constantly, until the mixture thickens and the butter melts. Stir in the cheddar cheese and dill until fully melted.
Grease a baking dish. Layer half the potatoes on the bottom, followed by half the cheese sauce, and half the salmon-spinach mixture. Repeat the layers, finishing with a final layer of potatoes and cheese sauce. Sprinkle the top with additional cheddar cheese.
Bake in the oven for 40 minutes, or until the top is golden and bubbly. Let it rest for 5 minutes before serving.
