Heat the oil in an oven-proof frying pan - mine measured 25cm in diameter - and add the sliced onions. Cook over a medium-low heat, stirring fairly regularly, for at least 20 minutes, until they are soft and golden brown. If you have time, you’ll get even more flavour into your onions by cooking them for even longer (up to an hour, if you have time!), at a lower temperature. I cooked mine for about 40 minutes.

When the onions are cooked to your liking, set the oven to 180°C (Gas Mark 4 / 350°F). Make a gap in the middle of the onions, and add the butter, allowing it to melt.

When the butter has melted, turn off the heat. Mix the butter into the onions, and spread them out evenly across the bottom of the pan. Pour the egg mixture on top.

Add a good handful of greens (I used rocket / arugula), and use a fork to press it down into the egg mixture. Dollop the ricotta cheese on top, and flatten it slightly with a fork, nestling it into the egg. You can add a little more black pepper to the top of your frittata if you like.

Place the pan into the oven, and bake for around 25-30 minutes, or until the eggs are fully set. They will set from the outside of the pan inwards, so as soon as the middle is set, it’s ready. Allow to stand for 5 minutes before cutting into slices.