Fill a large pan with the potatoes and water and bring to the boil. Simmer for 30 minutes or until cooked through. Drain and pass the potatoes through a ricer or a potato masher into a pan. Add the butter and cream and combine with the salt and pepper. Keep warm covered with cling film.
Meanwhile, heat a large frying pan, over high heat with a little oil and add in the burgers and cook for 2 -3 minutes on each side until browned all over, transfer the burgers onto a baking tray and put in the oven to finish for 10 minutes.
Place the pan back on the heat and over a medium heat add the onions and the herbs and allow them to sauté for a minute or two.
Next add in the cornflour and use this to coat the onions.
Gradually add the stock at this stage and allow it to come to the boil. Simmer gently for 3 - 5 minutes until it coats the back of the spoon. Check for seasoning also at this stage.
Serve the burger on top of the mashed potatoes and generously pour some of the onion gravy on the top. Serve immediately.