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Heat 2 tablespoons of the oil in a large non-stick frying pan. Fry the potatoes and onions on low heat and cover with a lid for 5 minutes to let them soften. Turn up the heat for another 5 minutes until golden brown. Remove from the stove and drain off any excess oil.
Meanwhile break the eggs into a medium bowl. Add the remaining salt and milk and whisk vigorously until frothy. Add the potatoes and onions to the eggs and mix until fully integrated.
Clean the frying pan and return to stove.
Heat the remaining 2 tablespoons of oil on a high heat and pour in the mixture, moving it around in the pan to help the tortilla to rise. Fry until golden brown, stirring occasionally until it has set. Then turn down the heat to allow the inside to cook. After a few minutes turn up the heat in order to brown the tortilla.
When it's cooked place a large plate over the frying pan and flip the tortilla onto it. Return to the pan and fry the other side until golden brown.