Use an oval baking dish which is approx. 28cm (11in) long by 19cm (7 1/2in) wide and about 5.5cm (2 ¼ in) high. But a round or rectangular dish would be good too. Grease bottom and sides of dish with butter.
Peel and cut potatoes and kumara in half and parboil until just soft.
Fry bacon til slightly crispy. Drain and chop finely.
Finely chop rosemary.
Let potatoes and kumara drain and cool slightly, for easier handling.
Cut potatoes into thin slices and put a layer of slightly overlapping potatoes on the bottom of a baking dish.
Sprinkle on some salt and pepper, bacon bits and rosemary.
Then pour on a generous amount of cream - about a third of a cup, so it is nicely covering the potatoes.
Do the next layer with kumara. And again the bacon, rosemary, salt and pepper and then cream.
The final layer is potato with rosemary, salt and pepper, any bacon that is left.
Pour on cream, brush with melted butter, then sprinkle green herb stock evenly across the surface.
Bake in the oven on 175 fan forced or about 190 regular oven for 1 hour.