Line a baking tray with greaseproof paper and drizzle with oil. If using a shallow skillet, drizzle with oil. Lay the potato slices in a single layer onto the tray or skillet, overlapping a little. Lay the sliced onion over the potato.
Stuff some parsley into the cavity of each fish, then lay them over the onions. Dot the sliced garlic over the fish, top with the cherry tomatoes, drizzle with a little more oil, then season well with salt and pepper.
Bake in the oven for 20-30 minutes, or until they feel soft and the skin starts to tear when touched. If you’re using larger fish, you may need to cut slashes into the sides and cook them for longer.
Serve straight from the oven with flatbread, sliced cucumber and salad leaves. Generously spritz with fresh lemon juice.