1. Finely slice the onion and cook gently in 10g of the butter until just starting to take some colour - about 6 minutes or so.
2. Meanwhile, finely slice the potatoes on a mandolin set to 1.5mm - don't be scared by this; it's a standard setting. If you don't have a mandolin, slice your potato as thinly as you can.
3. Butter the sides of a 25cm diameter casserole dish. Arrange a layer of potatoes over the base of the dish, scatter over the cooked onions and a sprinkle of salt.
4. Continue this way, alternating layers until you reach the final layer. You will get approximately 5 layers. Don't worry that the potato to onion ratio isn't fair - there will be many more potatoes.
5. When you reach the top layer, use the potatoes to make concentric circles so it looks pretty when you bake it. Pour over the stock; it should just come up to the top layer of potatoes but they shouldn't be 'swimming'.
6. Dot over the remaining 40g of butter in cubes and season well. The potatoes can be left for some time until you are ready to eat them at this stage.
7. When you are ready, bake in the oven at 180 degrees for around one hour or until the potatoes are cooked through and the top is browned nicely.