Preheat the grill. Boil 3 large potatoes (diced into 1cm squares) in salted water and drain. Fry a finely diced onion in some olive oil and butter with six slices of bacon, snipped into small pieces, until the onion is soft and the bacon browning slightly. Turn up the heat towards the end to crisp up the bacon, then reduce the heat to medium again. Add the drained potatoes and cook for a further 5 minutes. Add more oil if necessary.
Whisk up 6 large eggs with 3 tablespoons of milk and a good pinch of sea salt. Stir in a tablespoon of grated gruyère cheese. Pour the egg mixture into the frying pan, making sure that the potatoes are well distributed. Cook for about 4 minutes on the stove top, before sprinkling with a lot more gruyère and browning under a hot grill. Serve with salad and nice bread.