Thinly slice the potatoes and onion.
Cut the salmon into bite-sized pieces and thoroughly clean the spinach.
Melt butter in a pan, sauté the onion, then add spinach and cook slightly.
In an oven-safe dish, layer the potatoes, salmon, and spinach in order.
Mix together heavy cream, milk, salt, and pepper, then pour over the layers.
Evenly sprinkle mozzarella cheese on top.
Bake at 180°C (356°F) for 25 minutes for a perfect finish.