Preheat the oven to 200°C/Fan 180°C/Gas 6 and have ready a large baking tray with a rim (so no juices can escape). Tear off four large sheets of greaseproof paper or foil.
Cut the courgettes into thin ribbons or fine slices, using a mandolin, swivel veg peeler or a food processor fitted with a thin slicing blade. Put the courgettes into a large bowl. Ribbon or slice the carrots in the same way and add to the courgettes.
Finely slice the spring onions and add these too, along with the garlic, thyme, parsley if using, and two tablespoons of the olive oil. Season with a pinch of salt and a twist of pepper. Toss together well with your hands.
Pile the veg in the middle of each sheet of paper or foil, dividing it equally. Place a fish fillet on top of each pile and season it with salt and pepper. Gather up the edges of the paper or foil and bring them up around the veg and fish. Pour a good splash of wine, cider, stock or passata into each parcel.
Crimp the top edges of the parcel together. With foil, you’ll be able to seal the package completely; with paper, you can just scrunch it up so the fish is mostly covered. Place the parcels carefully on the baking tray. Bake in the oven for 15–20 minutes or until the fish is cooked through and the veg is just done (it will still be al dente).
Bring the parcels to the table and accompany with whole grains, to soak up the juices.