Heat a large skillet over medium heat and drizzle the bottom with 2 tablespoons olive oil.
Add onion and sautée for 8 to 10 minutes until the slices turn soft and translucent.
Then, add spinach directly from the package. Stir for a few more minutes until the leaves are fully wilted. When the onions and spinach are ready, sprinkle with salt and freshly ground nutmeg.
Meanwhile, in a large bowl, whisk together eggs with a pinch of salt and pepper. Stir in Ricotta cheese and Parmesan cheese.
Add the greens to the egg and cheese mixture and stir everything together.
Drizzle some more extra virgin olive oil (around 1 Tablespoon) to the same skillet you have used to cook the vegetables. This step is essential to avoid sticking.
Turn the heat on low and pour everything back in. Make sure the bottom of the pan is completely covered and that the ingredients are evenly distributed.
Cover with a lid and cook for around 10 minutes, until the egg on top is starting to set and the bottom is nice and firm.
Finally, flip the frittata with the help of the lid and cook it for 5 more minutes so that the other side also gets browned.
Serve hot or room temperature.