Peel and cut the onion in half. Cut each half into thin slices, roughly 1/8” thick.
Heat a 10” cast iron skillet over medium-low heat.
Add the olive oil, followed by the onions.
Sprinkle a little salt over the onions, reduce the heat to low, and cook until the onions are soft and starting to brown, 25 to 30 minutes or so.
Measure in the sherry vinegar, scrapping up any bits of onion stuck to the pan.
When the onions are almost done, preheat the oven to 425˚F.
Whisk together the eggs, heavy cream, salt, and black pepper; making sure to not overly beat the egg mixture.
Pour into the pan and let cook until the sides begin to set.
Sprinkle the goat cheese over the egg mixture and transfer the skillet to the oven.
Continue to cook for 10 to 12 minutes until the frittata is puffed and starting to brown on top.
While the frittata is baking, place the arugula in a bowl.
In a small bowl, whisk together the olive oil, lemon juice, lemon zest, mustard, black pepper, and salt. Pour over the arugula and toss to combine.
Once the frittata is done, let cool slightly. Top with the arugula and serve.
