Thinly slice the onions and the potatoes
Pour the milk into the vegetable stock and then add the butter - stir well. Season with salt & pepper. Add the dried herbs and stir well again.
Lightly grease the cast iron dish
Place a layer of sliced potatoes over the base of the dish, followed by a layer of onions, then potatoes etc until you reach the top of the dish
Pour the liquid over the potato mix and dab a couple of small knobs of butter over the top. You can place sprigs of fresh rosemary over the top if you prefer.
Place in the outdoor oven on a low heat (around 150 degrees) for an hour and then check with a knife to see if the potatoes are tender. You can top up the liquid if needed. If they need a little longer, pop the dish back in the oven until ready. If the potatoes are colouring up too much, place some foil over the dish and then remove this about 10-15 mins before you remove the dish to brown up.
2 medium white onions
1.5kg floury potatoes such as Maris Piper
400ml vegetable stock
50g butter (plus extra for topping)
Salt and pepper to season
1tsp rosemary + additional fresh sprigs to decorate if liked.