Heat sunflower oil in a heavy-based frying pan over medium heat, add bacon and sauté until golden (about 5 minutes). Using a slotted spoon, transfer bacon to an absorbent paper-lined plate. Reduce heat to low, add onion to pan and sauté until caramelised (about 15 minutes). Set aside.
Peel potatoes and coarsely grate. Place in a colander and wash in several changes of water. Place in a clean tea towel and squeeze dry.
Preheat oven to 200C. Lightly whisk yolks and combine with potato, bacon, onion and cream in a large bowl and season to taste with sea salt and freshly ground black pepper.
Whisk eggwhites to firm peaks, then gently fold through potato mixture. Spoon mixture into a well greased 2 litre-capacity baking dish and scatter over butter. Bake until golden (about 50-60 minutes) and serve immediately.