Heat oil in a large saucepan over medium heat.
Add sliced onions and potatoes and cook for about 15 minutes until softened and slightly brown. Place into a bowl and set aside.
Whisk eggs in a separate bowl and then pour over the potatoes and onions. Add salt and pepper.
Heat a small saucepan over medium heat with 1 teaspoon of oil.
Add egg, potato and onion mixture to the pan, adjusting the potatoes to make even layers.
Cook about 15 minutes until the egg on top is no longer runny.
Flip the tortilla over onto a large plate then slide back to the pan and cook another 5 minutes.
While the tortilla is cooking, combine the mayonnaise, grated garlic, and 1 tablespoon olive oil and whisk together to make the aioli.
Top with chopped parsley and serve alongside aioli.
