Cut four 30cm sheets of foil and lay on work surface. Cut 4 sheets of baking paper the same size and place on top of foil.
Divide spinach between parcels, then place a fish fillet on top. Divide zucchini and olives between parcels. Use a total of 2 tablespoons of the olive oil to drizzle onto fish. Season with salt and pepper.
Place remaining oil in a small bowl, whisk in lemon juice and rind, garlic and fresh herbs, then season with salt and pepper. Pour a little of the mixture over each fillet, bring up the edges and seal.
Bake for 10 minutes or until fish is cooked through – it will flake away easily when tested with a fork.
Drizzle with remaining dressing, and serve with boiled chat potatoes, if desired.