In a large skillet, heat the olive oil over medium-high heat. Add the sliced onions and sauté, stirring occasionally, until softened and golden around the edges, about 5 minutes. Add the garlic and potato slices and sauté, turning the potatoes and onions every few minutes, until the potatoes are cooked through and golden, about 10 minutes. (If you find the potatoes are getting too much color before they are cooked through, lower the heat to medium). Using a slotted spoon, transfer the potato mixture to a paper towel-lined plate. Reserve 2 tablespoons of the cooking oil and discard the rest.
In a medium bowl, beat together the eggs, salt and pepper. Add the potato mixture and gently stir to combine (it is okay if the potatoes break into pieces). Place a 9-inch skillet over medium heat and add the 2 tablespoons of reserved cooking oil. Pour the egg mixture into the skillet and using a fork, gently spread out the potatoes and onions so they are evenly distributed in the skillet. Cook, undisturbed, for about 3 minutes or until the tortilla is cooked around the edges but still loose in the center.
Carefully place a plate large enough to fit the tortilla over top of the pan and invert the tortilla onto the plate. Gently slide the tortilla back into the pan and cook on the other side for an additional 3 – 4 minutes or until the center of the tortilla is springy to the touch. Remove from the pan, cut into bite-sized squares and serve immediately.
If you like making tortillas, you may find it worth your money to invest in a set of 9 inch omelette pans that fit together. It makes inverting your tortilla a snap!