Preheat your oven to 200 degrees celsius
Thinly slice your kumara into disks, I used a mandolin for this but you can use a sharp knife.
Melt the butter, add the olive oil and rosemary to the melted butter and mix to combine.
Pour half of the butter/oil and rosemary mix into the bottom of a large baking dish.
Arange the slices of kumara into the baking dish so that they eventually sit almost vertically rather than lying flat.
Season the kumara with salt and pepper.
Brush the kumara slices with the remaining butter/oil.
Cover the baking dish with either a well fitted lid or tin foil and bake for 1 hour at 200 degrees.
Remove the lid or foil and bake for a further 10 minutes or so, so that the edges become a little golden.