With a microwave steamer, or on top of the stove, steam the broccoli and potatoes until almost tender. Set aside.
I n a large bowl, whisk by hand the eggs, salt, black pepper, and parsley. Set aside.
In a large skillet on top of the stove, heat the oil over medium heat.
Add the onions and cook them for a few minutes until they are translucent.
Add the egg mixture to the skillet.
Sprinkle the broccoli, potatoes, and cheddar cheese over the eggs, pushing them into the eggs with a rubber spatula.
As the bottom of the frittata cooks, lift the edges with a rubber spatula to allow the raw egg mixture on top to run underneath the bottom of the frittata. Keep doing this in several places.
Once it is mostly set but still just a little soft and runny, place a large dinner plate upside down on top of the frittata in the skillet. Place your hand flat down on the dinner plate and flip the pan over to get the frittata onto the plate.
Slide the frittata from the plate back into the skillet to let the runny side of the frittata cook on the bottom for a few minutes, or until golden brown.
Slide the frittata onto a clean serving plate.