Preheat the oven to 400°F.

Check the fish over and remove any pin bones. Divide it into two equal portions.

Lay out two sheets of aluminum foil, each about 12-14 inches long. Pile a large handful of baby spinach leaves in the middle of each piece of foil. Lay one fish fillet on each bed of spinach. Season generously with salt and pepper.

Lay a few slices of onion over the top of each fillet and scatter the cherry tomatoes on and around the fish. Lay two sprigs of thyme over top. Give each packet a squeeze of lemon and a drizzle of olive oil (or a pat of butter).

Fold the sides of the foil inwards around the fish. Then fold in the top and bottom of the foil and pinch them closed, creating a neat package. Set them side-by-side on a baking sheet and bake for 18 to 22 minutes, until the fish is opaque.

Open the packets carefully to avoid spilling the juices. Eat straight from the packets or transfer to a plate with a slotted spatula. Spoon some of the juices over top. Eat immediately.