Add olive oil to a deep 10" frying pan, add potatoes, chopped garlic, and red or white chopped onions.
Make sure there is enough oil to cover potatoes.
Saute uncovered until the potatoes are soft, but not brown.
Save a couple tablespoons, discard the rest.
A little before the potatoes are cooked, mix the eggs and milk.
When potatoes are cooked and drained, add them to the eggs and milk mixture.
Let sit for 10-15 minutes.
Season mixture with a bit of salt and lots of black pepper.
Heat a 10" non-stick frying and add the two tablespoons of olive oil.
Add the potato and egg mixture and cook on low heat until the egg is set (fork will come out clean if inserted).
Flip the tortilla using the lid of a large pan: Place lid over the frying pan; turn pan over; slide the tortilla cooked side up back onto the frying pan to finish cooking, just a couple of minutes longer.
Remove from heat and let tortilla rest for 15 minutes before serving.
May be served warm or at room temperature.
Serve in pie like slices or cut into cubes if serving it to a larger crowd.
Note: rather than flipping the tortilla you can cook on top of the stove until the egg is set on the bottom and then transfer the pan to a 400 degree oven to finish.