Preheat the oven to 180°C.
Heat 2 tablespoons of the olive oil in a large frying pan over medium-high heat and caramelise the onion. Don't overcrowd the pan or you'll end up with soggy onions instead of nicely caramelised ones - if you only have a small frying pan, work in two batches. Turn out into a bowl and set aside.
Spread the sweet potato on a baking tray, drizzle over 60ml of the olive oil and roast for about 1 hour, until soft, checking after 45 minutes. Remove from the oven and set aside to cool. Leave the oven on.
Heat the remaining 2 tablespoons olive oil ina frittata pan. Whisk the eggs well in a large bowl, then pour into the hot pan.
Dot the cooled onion and sweet potato all around through the egg, arranging a few onion rings on top. Do the same with the rocket and goat's cheese. Scatter the parsley through the frittata. Don't add salt until the end; salt in eggs while cooking can grey them a little.
Pop in the oven for 30-40 minutes, until there's no wobble in the middle. season with salt and serve fresh from the oven or cut into squares whenever food is needed. Because your vegies are packed neatly inside, slice into squares for lunchboxes, or it's a great stand-alone meal or, for an extra boost, add a salad alongside it.