Heat a very large (12-14 inch) nonstick skillet over medium-high heat, about 2 minutes. Add the olive oil.

Add the onion slices. Sprinkle them with a pinch of Kosher salt and a pinch of black pepper. Cook, stirring occasionally, until golden, about 5 minutes. Lower heat to medium.

While the onion is cooking, in a medium bowl, whisk together the eggs, 1/2 teaspoon Kosher salt, a pinch of black pepper, and 2 tablespoons Parmesan. Set aside.

When the onions are golden brown, add the spinach leaves to the skillet. Cook, stirring, just until beginning to wilt, about 1 minute.

Pour the egg mixture into the skillet. Cook the eggs, pushing them back and forth with a rubber spatula, until set to your liking. Sprinkle with red pepper flakes. Serve immediately.