In a large 10"-12" skillet, warm 1 Tablespoon extra virgin olive oil over medium high heat.
With a pinch of salt and pepper, sauté onion and garlic until translucent and beginning to brown, about 5-7 minutes. Remove from pan and set aside.
In same skillet, warm ½ of the oil (⅛ cup), over medium heat, then add potatoes, turning regularly until tender, but not mushy. Drain, reserving oil. Cool slightly.
In a medium size bowl, combine the eggs, milk (if using) with onions and potatoes.
Season with salt and pepper to taste.
Return reserved oil to coat the pan, plus the remaining ⅛ cup.
Warm oil over medium high heat.
Add egg/potato mixture and let sit and set, until a little soupy on top layer.
Continue to loosen around edges (and gently check underneath) to prevent sticking.
With a dinner plate, or platter larger than the diameter of the skillet, gently slide the tortilla onto the plate, still "wet" side up.
Using another same sized plate or platter, invert the tortilla onto 2nd plate. The cooked, browned side is now up.
Slide back into skillet, and cook until cooked through and brown on bottom, about 5-7 more minutes.