Preheat the oven to 180C/gas 4/fan 160C. Tip the spinach into a large hot pan, shake in 1 tbsp water and cook, stirring, for about 1 minute. Squeeze out any liquid. Or, if you have a microwave, pierce both bags of spinach and cook them together on Medium (500W) for 3 minutes, until the spinach is just wilted - it's more convenient but takes a couple of minutes longer.
Butter a baking dish and spread the spinach out on the bottom. Lay each plaice fillet out flat on a chopping board, skinned-side up, and season with salt and pepper. Roll each one up like a Swiss roll and arrange on the spinach.
Spoon the sauce over the fish and spinach so they are well covered. Scatter over the almonds and bake for 20 minutes, until the fish is cooked and everything is golden and gently bubbling. Serve with boiled potatoes.