Preheat the oven to 200C/Gas6. Cook the potatoes in lightly salted boiling water for 15-20 minutes, until just tender; drain and set aside to cool; shred half the spinach
Thinly slice the potatoes lengthways; brush a baking tray with a little oil; lay a sheet of filo pastry on kitchen work top, brush it with oil, then lay a second sheet crossways over the first; repeat with two more sheets.
Arrange a quarter of the sliced potatoes in the centre of the pastry; season well and add a quarter of the spinach, a quarter of the garlic and some lemon rind; lay a fish fillet on to, skin side up and season well
Loosely fold the filo pastry up and over to make a neat parcel; make three more parcels in the same way; place on the baking tray and brush them with half the butter; bake for 20 minutes until the filo is puffed up and golden brown
To make the spinach sauce, heat the remaining butter in a small pan, add the reserved spinach and cook until it wilts; stir in the fish stock and cream; heat almost to boiling point stirring constantly; season, blend and keep hot
Serve the fish parcels garnished with the red pepper and with the spinach sauce on the side