Heat oil in a non-stick pan and sear the potato and onion over medium-high heat, for about 4 minutes. Next, add the zucchini and sauté for another 4 minutes.
In a bowl, whisk eggs and season with salt and pepper. Transfer the vegetables from the pan into the bowl and mix well.
Using the same pan, add the egg mixture on low heat and make sure everything is evenly distributed. After about 3 minutes, run a spatula through the outer edges of the tortilla to make sure it does not stick to the pan.
After 8-10 minutes, flip the tortilla (this might take more or less, depending on heat, size and pan), using a plate over the pan. Slide the uncooked part back into the pan.
After another 5-6 minutes, the tortilla should be cooked. Remove from heat and serve.