Preheat the oven to 200°C.
Cut the potatoes and leek into 1cm slices and lay them in a baking tray. Brush them with oil, season with salt and pepper then bake for 20 minutes.
Meanwhile halve tomatoes and chop the garlic.
Take the tray out of the oven and toss the potatoes with the tomatoes, thyme and garlic. Drizzle with a little oil.
Roast for 10 minutes, then put the fish on top, season with salt, then scatter the green vegetables capers around the fish. Drizzle with oil, then bake for another 10 minutes until the fish is cooked through.
Serve with freshly ground black pepper and chopped herbs scattered on top.