Preheat the oven to 200c°C.
Cut the potatoes into half cm slices and lay them in a baking tray. Brush them with oil, season with salt and pepper then bake for 20 minutes.
Meanwhile thinly slice the onion, garlic and red pepper. Cut the tomatoes into 4 pieces.
Take the tray out of the oven and toss the potatoes with the tomatoes, thyme, garlic, red pepper and onion. Drizzle with a little oil.
Roast for 10 minutes, then put the fish on top, season with salt, then scatter the capers around the veggies, together with the olives, if used.
Bake for another 10 minutes or so, until the fish is cooked through properly.
Serve with freshly ground black pepper and chopped herbs scattered on top.