1Preheat your oven to 375°F (190°C).
In a large oven-safe skillet, heat the olive oil over medium heat.
Add the diced onion and cook until it becomes translucent, about 5 minutes.
Add the sliced zucchinis to the skillet and cook until they are tender, about 5-7 minutes.
In a large bowl, whisk together the eggs, salt, and black pepper.
Pour the egg mixture over the cooked onions and zucchinis in the skillet.
Sprinkle the crumbled feta cheese evenly over the top.
Cook on the stovetop for about 2-3 minutes, until the edges start to set.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is fully set and golden on top.
Remove from the oven and let it cool for a few minutes before slicing and serving.