If preferred the potatoes can be finely peeled or the skins can be removed once cooked.
Boil the potatoes for 10-12 minutes until just tender.
Meanwhile heat 1 tablespoon of the oil in a small frying pan and cook the onion over a moderate heat until softened and golden.
Increase the heat slightly and add the garlic and mustard seeds, cook for 1-2 minutes, stirring, the mustard seeds will pop during cooking. Remove from the heat and set aside.
Drain the potatoes and slice thickly.
Heat the rest of the oil in a very large frying pan. Add the potato slices in a single layer, if the pan is not large enough cook in two batches, keeping the first batch hot in the oven.
Fry the potato slices over a moderately hot temperature for 5-7 minutes turning them until crisp and golden on each side. Lift the sautéed potatoes into a warm bowl lined with kitchen paper, to drain them. Remove the paper, add the onion mixture and toss together.
Crush some flaked salt over and serve.