Place eggs, heavy cream, chopped spinach, chopped herbs, Kosher salt, and black pepper in a medium bowl. Add diced red onion, then cook until translucent, stirring occasionally. Add chopped garlic and cook 1 minute more.
Reduce the temperature to medium-low, then pour the egg mixture on top of the onions. Cook, then transfer the skillet to the preheated broiler. Cook until the top is set and golden brown.
Remove the skillet from under the broiler. Sprinkle with a pinch of flaky sea salt. Optional: serve with crumbled goat cheese or feta cheese, if desired.