Preheat oven to 350°F / 180°C

Place the potato and onion in a roasting pan lined with parchment paper. Drizzle with 1 1/2 tablespoons oil and sprinkle with 1/2 teaspoon salt. Toss well. Bake for 1 hour or until the potatoes are tender, flipping occasionally with a spatula to prevent sticking and promote even browning.

In a large bowl, whisk the eggs together with 1/4 teaspoon salt. Stir in the potato mixture; let stand for 5 minutes in order to give the potatoes a chance to absorb some of the egg mixture.

Using a 20 cm non-stick skillet, heat the remaining oil over medium heat. Transfer the potato mixture into the pan (the pan will be very full so use a spatula to flatten down the potatoes a little). Cook 7-8 minutes (until almost set), gently and frequently shaking the pan so the tortilla doesn't stick.

Place a large plate upside down over the tortilla and quickly invert it onto the plate. Carefully slide the tortilla (cooked side up) back into the pan, so the other side cooks; carefully tuck in the sides with a fork, and continue cooking, occasionally shaking the pan until eggs are set, about 3 minutes.

Carefully loosen the tortilla with a spatula; gently slide it onto a plate. Cut into wedges and serve at room temperature. Enjoy!