Preheat the oven to 200C/Gas 6.
In a roasting dish, scatter the potatoes, lemon slices, onion, garlic, rosemary and thyme and drizzle with oil. Season well with salt and black pepper, cover loosely with baking parchment and place into the oven to roast for 25 minutes, or until the potatoes are tender.
Remove from the oven, then turn up the heat to 220C/gas 7.
Season the fish all over with salt and pepper, then stuff a few sprigs of herbs into the cavity. Place the fish on top of the lemons and potatoes and add a few dollops of butter to the dish. Give everything a good drizzle of oil and return the dish to the oven and roast for 18-20 minutes.
Remove the roasting dish from the oven, scatter some roughly chopped dill across everything and serve at the table.