Preheat the oven to 180C. Grease a springform 8 inch cake tin.
Peel, half and then finely slice the onions. Place the onions in a pan with the herbs and the butter and oil. Let this all sweat down for about 15 minutes on a low heat, making sure they don't brown or stick.
Peel the potatoes and using a mandolin.
Starting from the middle, start laying the potatoes from the middle, fanning them out to the edge. Make sure they overlap slightly.
Once the whole layer is done, spoon on some onions in an even layer. Grate in the cheese. Continue to layer the cake in the same format, seasoning as you go. Make sure there is enough cheese and onions for each layer.
Place in the oven and cook for around 30-40 minutes, until the potatoes and soft, cooked and golden. Slice like a cake.
