In a deep sided 8" x 8" pan, add baking parchment to stop the potatoes from sticking. In order to allow the potatoes to be lifted out, place one sheet of baking parchment in the pan, allow the parchment to come well up over the sides to form "handles" to lift the cooked dish out. Place a second piece of baking parchment in the opposite direction of the first but this only needs to cover the sides and the base. NOTE: Ensure the piece of parchment with "handles" is on the bottom of the pan.
If using a mandolin, set it to approximately 2-3mm thickness. [If you don't have a mandolin, use a very sharp knife instead.] Peel and slice the potatoes thinly and place into a large bowl with cold water to stop them browning while you prepare the dish.
In a small saucepan, add the butter, vegetable stock, thyme leaves and garlic. Allow the butter to melt but not boil. Turn off heat and get ready to layer the dish.
Start layering the potatoes in a single layer, only overlapping slightly on the bottom of the pan. Using a pastry brush, brush on some of the butter and stock mixture. Next, season well with salt and pepper. Repeat this process layer by layer, topping each potato layer with the butter mixture and salt & pepper until you reach the top of the pan or the potatoes are finished.
Once the dish has been fully layered, cover with tin foil and bake in the oven for 1 hour. Check the potatoes after an hour by inserting a skewer in the middle. They are cooked if the skewer passes through with ease. If not, continue to cook another 15-20 minutes.
Once fully cooked, remove from oven. Remove the foil and replace with a piece of baking parchment to fit over the top but inside the pan. At this point you want to weigh down the potatoes while still warm. You can use a small plate or a piece of cardboard to put on top of the baking parchment, inside the pan. Top with tinned goods to form a weight. Allow to fully cool like this before placing in the fridge overnight to set.
When ready to use, remove the pan from the fridge and using the "handles" of baking parchment as mentioned in Step 2, gently remove from the potatoes from the pan. They will be heavy and might need a little rocking motion to release them.
Once out of the pan, slice in half and then each half into 6 portions. This will give you 12 equal portions.
Place each portion on a lined baking tray, you can top with a small knob of butter each and place in a preheated oven at 180 fan uncovered for 20 minutes to warm up and allow to crisp. Ensure piping hot before serving.
This dish can be prepared up to 2 days before serving. It can also be made into smaller batches, just adjust accordingly.