Heat your oven to 200ºC/Fan 180ºC/Gas 6. Wash the fennel. Peel the carrots. Slice both finely (use a mandoline if you have one). Rinse the broccoli. Chop it into small, thin florets.
Pop the veg in a bowl. Grate or pare the zest of the lemon. Save the lemon for step 3. Add the lemon zest, the olives, 2 tbsp olive oil and some salt and black pepper to the bowl of veg. Mix to coat.
Cut two large sheets of baking parchment. Divide the veg mix between them. Pop the cod fillets on top. Slice the lemon in half, cutting a round from each half and pop on top of the fish. Squeeze each remaining half of lemon over each parcel. Season.
Bring the edge of the paper together to make a parcel (see our tip). Pop the parcels on a baking tray. Bake in the oven for 20-25 mins till the fish is cooked and the veg are tender. Fill the kettle and boil it.
Scrub the potatoes. No need to peel them. Slice them into discs. . Fill a pan with hot water from the kettle. Pop the potatoes in the boiling water. Simmer for 5-10 mins till tender. Drain. Pat dry with kitchen paper.
Heat 2 tbsp olive oil in a large frying pan. Add the potato slices in batches, so the pan isn't crowded. Fry for 2-3 mins till they are pale golden. Pop on a plate lined with kitchen paper. Repeat till all the potatoes are cooked. Keep warm in the bottom of the oven.
Rinse and pat dry the [arsley. Finely chop the leaves. Take the parcels out of the oven. Check the fish is done. It should easily flake and be opaque. Serve the parcels with the sauté potatoes. Scatter the parsley over the top.