Blanch the chopped asparagus in a pot of boiling water for 2 minutes. Drain and pat them dry.
Heat the extra-virgin olive oil in an oven-friendly skillet over medium-low heat, add the onion (if using) and asparagus and saute for 5 minutes, until slightly softened.
In the meantime, beat the eggs with the Parmesan and ricotta cheese in a large bowl, then season with a generous pinch of sea salt and freshly-cracked black pepper.
Distribute the asparagus so that they cover the bottom of the skillet evenly. Pour the egg mixture over the vegetables, cover with a lid and cook over low-medium heat for 5 minutes. When the frittata is easily pulled off from the bottom but the surface is still jiggly, it is ready to go in the oven.
Remove the lid, and sprinkle 1 extra tabslepoon of Parmesan on top and chives if you like. Transfer the skillet in the oven under the grill for 5 minutes or until golden and cooked through. Cut into wedges and serve.
You must use a 9-inch or a 10-inch oven-safe skillet to make this frittata. If you do not have an oven-safe skillet, saute the asparagus, then transfer the veggies to a 9 or 10-inch cake pan or baking dish, pour the egg mixture over the vegetables and bake. You may need to add 5-10 minutes to bake time.
Frittatas can be served warm, cold, or at room temperature. They are safe to leave for max 2 hours at room temperature.
Store leftovers in an airtight container for up to 4 days in the refrigerator.