Heat 1 tablespoon of the olive oil in a medium frying pan, with a metal handle, over a low heat and add the onions and garlic. Cook for 5 minutes until golden, then add the cooked potatoes and heat through.
Meanwhile, in a large bowl, beat together the eggs and milk. Add the potatoes, onion and garlic to the egg mixture and stir well.
Return the pan to the heat, and heat the remaining oil. Tip the potato and egg mixture into the pan and cook over a low heat for 7–8 minutes, until beginning to set. Preheat the grill to a medium heat and cook the tortilla in its pan under the grill for 3–5 minutes until the top is golden and set.
Turn out the tortilla on to a plate and allow to cool. Cut into slices and serve warm or cold. (You can wrap any unused tortilla securely and refrigerate it – eat within 3 days.)