Peel the potatoes and cut them into thin slices, ensuring all uniform in size to ensure even cooking.
Cut the onions, the same way.
Place the potatoes in a bowl and wash them to remove excess starch.
Place in a colander and leave them until ready to use
Heat oil in a frying pan.
Add potatoes and onion to the hot oil and fry them over medium-high heat. When they begin to brown, reduce heat to low and stir to prevent sticking.
While the potatoes and onion are cooking, beat the eggs in a large bowl with the milk and salt to taste. add a pinch of salt to each egg. Beat until smooth.
When the potatoes and onion are ready, remove them from the oil with a slotted spoon and drain well.
Add them to the beaten egg mixture, adding a tablespoon of oil for each egg.
Combine the onion/potato mixture with the eggs
Mix well and let sit for about 10 minutes
Add two or three tablespoons of the oil used to fry the potatoes to a skillet and heat on medium
When it's very hot, pour the egg, potato, and onion mixturei nto the pan.
Using a spatula or pastry scraper, stir gently at first to distribute the uncooked egg.
Set the tortilla: Place a lid on the pan and cook over medium-low heat until the edges of the tortilla are set.
Place a plate over the pan, carefully flip it over, and slide the tortilla back into the pan to cook the other side.
For a lightly set tortilla, cook over medium-high heat for about 5-8 minutes per side to ensure it browns quickly and retains its juiciness;