Heat about a quarter of the butter in a
large pan, set over a medium heat. Sweat
the onions with a little salt and the
rosemary sprig. Slowly cook on a gentle
heat with a lid on for about 20 mins.
Remove rosemary and set aside.
Heat oven to 190C/170C fan/gas 5. Peel
the potatoes and immediately cut into
wafer-thin slices directly into a bowl
containing the rest of the warm butter.
Season with salt and pepper. The thinner
the potatoes, the better the dish will be.
Do not wash them once they are sliced,
as you want all of the starch to stick the
Line an ovenproof, non-stick frying
pan with a round of baking parchment.
Layer potatoes neatly into the dish in a
circle, slightly overlapping. The bottom
of the pan will become the presentation
side. Continue to pile in half of the
potatoes, then add the melted onions.
On top of the onions continue with the
potatoes until all used up. Set the pan
over a medium heat and gently begin to
colour the bottom of the dish for 5 mins,
then transfer to the oven and cook for
45-50 mins until the potatoes are soft.
Remove from the oven and leave to
rest for 10 mins. Invert onto a serving
plate, then slice into wedges with a
very sharp knife.