Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a small and deep roasting tin with greaseproof paper.
In a non-stick frying pan, heat the olive oil and fry the onions until very soft.
Meanwhile, preheat the grill to medium.
When the onions are translucent, add the spinach in gradually until it has wilted down then stir well to combine.
In a large bowl, whisk the eggs with a generous amount of salt and pepper.
Top the spinach mixture with the goat’s cheese.
Pour in the egg mixture into the prepared tin evenly.
Cook until the sides have started to set, about 15-20 minutes.
Preheat the grill to a medium setting.
Transfer the tin to the grill and let the frittata set, go golden and puff up slightly.
Shake the tin gently to check it has set.
If it still looks runny, return to the grill for a few more minutes.
Remove from the grill, allow it to cool slightly then use a palette knife to release the sides.
Slide the frittata onto a plate or board and serve sliced into wedges.