Preheat your oven to 400°F. This is crucial as we want the frittata to bake perfectly once it’s on the move.
In a 9-inch or 10-inch oven-safe non-stick skillet, heat the olive oil over medium heat. Watch as it shimmers, signaling it’s ready for the next step.
Add the finely chopped red onion and scallions to the skillet. Cook until they become soft and fragrant, about 4 minutes. Keep stirring occasionally to prevent sticking.
While the onions and scallions are cooking, crack the eggs and egg whites into a medium bowl. Beat them together until well mixed and slightly frothy.
Next, add the kosher salt, freshly ground pepper, feta, and Parmigiano-Reggiano to the mix. Stir in the sautéed onions and fresh spinach, making sure everything is well incorporated.
Pour this mixture into the skillet with the sautéed onions and scallions. Allow it to cook on the stovetop until the bottom begins to set, about 4 to 5 minutes. Keep an eye on it, as this is where the magic begins!
Once the bottom sets, transfer the skillet to the preheated oven. Bake for an additional 4 to 5 minutes, or until the frittata is puffed and golden on top. The aroma will be irresistible!
After baking, carefully remove the skillet from the oven. Let it cool for a minute, then transfer the frittata to a plate. Cut it into 4 wedges for serving.
For a finishing touch, serve the frittata topped with diced tomatoes and extra scallions if desired. Enjoy every delicious bite!