Place half the breadcrumbs in a blender or food processor with the chopped parsley and garlic and blend together until the breadcrumbs are slightly green. Mix these breadcrumbs into the remaining crumbs, together with the lemon rind, walnuts and cheese. Season with salt and pepper and mix together well.
Sprinkle a little seasoned flour over each plaice fillet, then dip the flesh side of each fillet into the beaten egg. Place the fillets, skin side down, on a baking sheet and sprinkle the breadcrumb mixture over the fish. Melt the butter with the olive oil and drizzle over the top. Cover and chill until required.
When ready to serve, remove the covering and cook the fish on the top shelf of a preheated oven, 200°C (400°F), Gas Mark 6, for 12–14 minutes, or until the crust is golden brown and the fish cooked through. Serve with wedges of lemon, fresh herbs, matchstick chips and a green salad.