Begin by slicing the onion thinly. Melt butter in a skillet over medium-low heat and add onions with sea salt. Sauté for 25-30 minutes until caramelized.

Stir in sherry vinegar once onions are caramelized, scraping the skillet's bottom to enhance the flavor.

Preheat oven to 425°F (220°C).

In a mixing bowl, whisk together eggs, heavy cream, additional salt, and black pepper until just combined.

Pour the egg mixture over the sautéed onions and cook briefly on stove for 2-3 minutes until edges begin to set.

Sprinkle crumbled goat cheese over the partially set egg mixture.

Transfer skillet to the preheated oven and bake for 10-12 minutes until puffed and slightly browned.

While baking, combine arugula with olive oil, lemon juice, lemon zest, and mustard in a bowl. Toss to coat.

Once the frittata is slightly cooled, slide it out onto a cutting board, top with arugula salad, slice, and serve warm.