Preheat oven to 400°F (200°C). Heat olive oil in a 10-inch oven-safe skillet over medium-low heat. Add the sliced onion and cook until slightly browned and caramelized, about 10-12 minutes. Stir occasionally.
Add the diced red bell pepper to the skillet and cook for another 5 minutes, until softened.
In a medium bowl, whisk together the eggs, grated Parmesan cheese, salt, and pepper until well combined.
Pour the egg mixture evenly over the vegetables in the skillet, ensuring it covers them. Cook on the stovetop for about 2 minutes, or until the edges of the frittata begin to set.
Transfer the skillet to the preheated oven. Bake for 10-15 minutes, or until the frittata is set and lightly golden brown on top.
Let the frittata cool slightly before cutting into 4 wedges and serving warm.
Notes
This frittata is versatile and can be enjoyed for breakfast, lunch, or dinner. Feel free to add other vegetables like spinach, mushrooms, or zucchini for extra flavor and nutrition.