Heat the olive oil in a large (12-inch) nonstick skillet over medium-high heat for about 2 minutes.
Add the onion slices. Sprinkle them with a pinch of kosher salt and black pepper. Cook, stirring occasionally, until golden, about 5 minutes. Lower the heat to medium.
While the onion is cooking, whisk the eggs, ½ teaspoon of kosher salt, a pinch of black pepper, and 2 tablespoons of parmesan in a medium bowl. Set aside.
When the onions are golden brown, add the spinach leaves to the skillet. Cook, stirring, just until the leaves begin to wilt, about 1 minute. Don't overcook the spinach.
Pour the egg mixture into the skillet. Cook the eggs over medium heat, pushing them back and forth with a rubber spatula, until set to your liking.
Sprinkle the scramble with red pepper flakes. Serve immediately.
It's best to use bagged pre-washed spinach in this recipe. For the best texture, you want the leaves to be completely dry, and the pre-washed product ensures that.
To prevent overcooking the eggs, it's important to cook them over medium heat.
Provided the eggs are fully cooked and no longer liquid, you can keep the leftovers in the fridge, in an airtight container, for 2-3 days. Reheat them gently, covered, in the microwave at 50% power.