When dinner needs to be light but still pack protein, this egg white frittata with spinach and feta delivers. Works just as well for breakfast — versatility is the point.
Preheat your oven to 375°F (190°C) and set a 10-inch oven-safe nonstick skillet on medium heat.
Add olive oil to the skillet, then stir in finely chopped yellow onion; sauté 3-4 minutes until the onion is translucent but not browned.
Add fresh spinach, tossing frequently, and cook until just wilted (about 2 minutes); increase heat slightly if necessary to evaporate any excess moisture.
Lower heat to medium-low. Pour in egg whites evenly over the vegetables, spreading them gently with a spatula; sprinkle feta cheese evenly across the top.
Let the mixture cook undisturbed for 2-3 minutes on the stove just until the edges set, then transfer the skillet to the oven and bake for 15-18 minutes or until the center is set and the frittata is puffed but not browned.
Remove from oven, let cool for 3 minutes before slicing. Serve warm or at room temperature.