Cook the potato cubes in boiling water for 15 minutes. Drain.
Preheat the oven to 190ºC, fan 170ºC, gas 5. Cook the spinach in a large pan over a high heat for a few minutes until wilted. Drain and squeeze as much water out of the spinach as possible. Set aside to cool.
Mix the soured cream, spring onions and lemon zest with some seasoning.
Take 2 x 2 litre ovenproof dishes and divide the potatoes and spinach evenly between them. Lay the fish on top, then spread over the soured cream mixture, dividing between the two dishes. Mix the breadcrumbs and Gruyère and sprinkle over both dishes.
Bake one dish for 35-40 minutes. To freeze the other dish, cool it completely and freeze for up to 1 month. Defrost and cook as above or cook from frozen, doubling the cooking time. Check that it's piping hot in the middle before serving.