Heat 2 tablespoons of oil in a 24cm non-stick pan over medium heat. Add the sliced potatoes and cook for 8 to 10 minutes, stirring often, until they start to soften. Keep the heat moderate so they don’t burn.
Add the sliced onion and continue cooking for another 5 minutes, stirring often, until the onion is soft and lightly golden. Check the potatoes are tender by piercing with a knife. If not, cook a few minutes longer.
In a bowl, whisk the eggs with a good pinch of salt until well combined. Pour the eggs evenly over the potato and onion mixture, tilting the pan slightly so it spreads into all the gaps. Season the top well with pepper.
Reduce the heat to low and cook for 8 to 10 minutes until the edges are set and the centre is just slightly soft. Run a spatula around the edges to loosen it from the pan.
Place the pan under a preheated grill and cook until the top is set and lightly golden. Alternatively, place a plate over the pan, carefully flip the frittata out, then slide it back into the pan to cook the other side.
Remove from the heat and let it rest for a few minutes before slicing. This helps it firm up and hold its shape. Cut into wedges to serve. Garnish with extra sea salt before serving.