Preheat the slow cooker if necessary; see the manufacturer's instructions. Beat the butter with the lemon rind and a little salt and pepper.
Lay the trout fillets on a chopping board so that the skinned sides are uppermost. Trim the edges to neaten, if needed, and spread with half the lemon butter. Top with the plaice fillets, skinned side uppermost, and spread with the remaining butter. Roll up each fish stack, starting at the tapered end. Secure each spiral with 2 cocktail sticks at right angles to each other and arrange in the base of the slow cooker pot.
Pour the lemon juice and boiling stock over the fish and add a little salt and pepper. Cover with the lid and cook on low for 1½–2 hours or until the fish flakes easily when pressed in the centre with a knife.
Lift the fish spirals out of the slow cooker pot with a slotted spoon, put on to a serving plate and remove the cocktail sticks. Strain the cooking juices into a bowl. Put the egg yolks in a saucepan and gradually whisk in the strained stock until smooth. Cook over a medium heat, whisking constantly without boiling for 3–4 minutes or until lightly thickened and foamy. Pour into a jug. Serve the fish with a generous drizzle of the sauce around each one, salad and a separate bowl of tiny new potatoes.