Heat olive oil in a pan over medium heat. Place the potatoes and onion in the pan and cook over medium low heat until the potatoes are fork tender. Stir occasionally to make sure they cook evenly.
Once the potatoes are fork tender, transfer the potatoes and onions to a bowl and discard the excess oil.
In a large bowl, whisk the eggs until incorporated. Add in the potatoes and onion and stir to combine. Season with salt and pepper.
Heat 2 tbsp olive oil in the same skillet. Pour in the potato and egg mixture into the pan.
Cook over medium heat for a 8 to 10 minutes until the bottom is golden brown.
Place a large round plate on top of the pan and hold it firmly. With the other hand, hold the skillet’s handle and quickly but firmly, flip the omelette into the plate (you can do this over the sink to avoid any leakage).
Place the pan over the heat again and slowly, push the omelette into the pan and cook over medium heat for another 8 to 10 minutes. IF you like your tortilla softer and runnier, cook for 6 to 8 minutes.
Once the other side is golden brown as well, the Spanish omelette is ready to serve.
Mix the tomatoes, cucumber, onion, olive oil, lemon juice and salt in a small bowl and serve it with the Spanish tortilla.