Wash and peel potatoes. Thinly slice using a grater, mandolin or knife.
Heat oil on medium high in a non-stick frying pan (10' is good). Test for proper temperature by adding a piece of onion. If it sizzles then it is ready. Add onions to oil, then add potatoes. Turn the potatoes as they cook.
Crack the eggs in a medium sized bowl with salt and pepper. Add milk. Whisk eggs and milk.
Potatoes should begin browning. Turn every few minutes. When potatoes are beginning to brown and are tender, drain off the extra oil and reserve.
Add drained potatoes and onion to bowl with whisked eggs and milk.
Add some reserved oil to the pan. Pour egg, potato and milk mixture into the pan.
Cook eggs on a medium heat. Make sure that eggs are not sticking to the pan by shaking. You should see bubbles coming up. Once you are sure that the eggs have set on the cooked side, and the eggs have begun to set on top, you can flip it. Cooks note: If you don’t want to flip the tortilla you can finish the cooking in the oven on broil if the frying pan is oven safe.
To flip the tortilla, cover pan with a plate that is bigger than the pan. Flip the pan with the plate held tightly to the pan. The tortilla should come off onto the plate. Slide the tortilla back into the pan to cook the other side.
Once the egg is firm and cooked through, flip the pan with a plate held firmly to the pan. The tortilla should come out right onto the plate. Serve on the plate.