Start by taking a non-stick skillet and sautéing your sausages in a bit of olive oil over moderate heat. When they are nicely browned, you can remove them to prevent them from getting too brown, or just keep them in the pan.

Now add your onion and potato and let them go until they are nice and tender, and not too brown. Season with salt and pepper as they sauté. I find that it helps to add some water and cover the skillet for a few minutes. This speeds up the cooking time and prevents burning.

Once the potato and onion mixture is done, add back the sausage pieces if you've removed them. Let them sauté together and get to know one another for a minute or two. If you are using the parsley, now is the time to add it to the mixture.

While the filling ingredients are getting acquainted, it's time to make your egg base: Beat the eggs and grated cheese together in a bowl. If you are using pecorino, you should not need much salt, but with parmesan you will want to season the eggs a bit.

Pour the egg mixture over the filling ingredients in the skillet and mix once with a spatula or wooden spoon.

Once the flavoring ingredients are more or less evenly distributed though the eggs, stop and let the eggs cook, still over moderate heat, until they have almost entirely set, except for a thin layer of raw egg on top, and the bottom has nicely browned (but not too much). During this step, too, you can cover the skillet with a lid to help the eggs set. The whole process should not take more than 8-10 minutes. Use a spatula to check the bottom to make sure it's browning nicely but not burning. Regulate the heat if need be.

Now comes the tricky part: you need to flip the frittata over. I do this by using two plates. You slip the frittata out, cooked side down, onto a dinner plate. Then top that plate with another, and flip the two plates over, so that the frittata is now upside down, with the runny, uncooked side facing up.

Add a little more olive oil into the empty skillet, then slide the frittata back into the skillet. You will now want to let the runny side cook. This will take a lot less time than before, as you eggs should have already almost completely set, perhaps 3-5 minutes more. Again, check the bottom by raising a bit of the omelette with a spatula and take a peek underneath. It may look a bit misshapen at this point, but not to worry, all will be well in the end.

When the frittata is done, slide it out onto a dinner plate again. I find that the last-cooked side is the most attractive, so once again, I use another plate and flip it over onto its 'pretty' side.