Preheat the oven to 200℃ (180℃ fan)/400°F/gas mark 6.
Place the potatoes in a roasting tin and drizzle with 1 tbsp of the oil. Sprinkle with a little salt. Roast for 15 minutes.
Meanwhile place the remaining oil, onion, tomatoes, capers parsley and vinegar in a food processor and process to chop finely. Alternatively chop the onion, caper and parsley finely by hand and combine with the oil and vinegar.
Season the cavity of the fish with salt and pepper.
After the potatoes have cooked for 15 minutes, turn the potatoes over, then add the trout to the tin and cook for a further 15 minutes.
To serve, remove trout and keep warm, lightly crush each potato with a fork, then add the onion and parsley mixture to the tin and toss to coat the potatoes. Serve immediately alongside the trout.
The potatoes also make a delicious side dish for grill fish or meat and can be served hot or cold.Nutrition information is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)