Start by heating the olive oil in your oven-safe skillet over medium heat. Once the oil is shimmering, add the thinly sliced white onion. Sauté the onions, stirring occasionally, until they are golden brown and caramelized, about 10-15 minutes. Season with a pinch of salt to help draw out the moisture and enhance the sweetness.
When the onions are perfectly caramelized, add the diced red bell pepper and zucchini matchsticks to the skillet. Cook for an additional 5-7 minutes, until the peppers are tender and the zucchini has softened slightly. Season with salt and fresh pepper to taste.
In a mixing bowl, crack the six large eggs and whisk them together until well combined. Stir in the grated Parmesan cheese, along with a pinch of salt and pepper.
Pour the egg mixture over the sautéed vegetables in the skillet. Gently stir to ensure the vegetables are evenly distributed throughout the egg mixture. Allow it to cook undisturbed for about 3-5 minutes on the stovetop until the edges begin to set.
Preheat your oven to 375°F (190°C). Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set in the center and the top is lightly golden.
Once baked, remove the frittata from the oven and let it cool for a few minutes. Slice it into wedges and serve warm or at room temperature. It pairs beautifully with a light salad or some crusty bread for a heartier meal.
