Preheat oven to 180C. Combine 100gm butter and sugar in a medium saucepan, add onion and cook over low heat for 6 minutes or until soft, then increase heat to medium and cook for another 18-20 minutes or until caramelised. Season to taste with sea salt and freshly ground white pepper.
Using a mandolin, slice potatoes 3-4 mm thick, then arrange in a single layer in the base of a 2-litre capacity baking dish. Scatter with some of the onion mixture, then repeat with remaining potatoes and onion and season to taste between layers, finishing with potato.
Meanwhile, bring chicken stock to the boil in a small saucepan. Pour over potatoes until just covered, then add remaining butter. Bake for 50-60 minutes or until potatoes are tender.