01 - Preheat the oven to 350°F.
02 - Heat olive oil in a large ovenproof skillet over medium heat. Add diced red onion and red bell pepper; sauté for 3 to 4 minutes until softened.
03 - Add chopped spinach to the skillet and cook for 1 to 2 minutes until wilted. Stir in halved cherry tomatoes.
04 - In a mixing bowl, whisk together eggs, cottage cheese, grated Parmesan, chopped chives, dried oregano, black pepper, and sea salt until well combined.
05 - If using, add diced cooked chicken breast to the skillet. Pour the egg mixture evenly over the vegetables and chicken.
06 - Cook on low heat for 3 to 4 minutes until the edges start to set.
07 - Transfer the skillet to the oven and bake for 15 to 18 minutes until the center is fully set and lightly golden.
08 - Allow the frittata to cool for 5 minutes before slicing and serving.