01 - Preheat your oven to 375°F.
02 - Heat olive oil in an ovenproof skillet over medium heat.
03 - Add red onion and bell pepper; sauté for 2 minutes.
04 - Add zucchini and cook for another 2 minutes, stirring occasionally.
05 - Stir in garlic and spinach, cooking until spinach wilts, about 1 minute.
06 - Add the diced chicken and cook for 1 minute to warm through.
07 - In a bowl, whisk together eggs, milk, salt, pepper, oregano, and thyme.
08 - Pour egg mixture evenly over the chicken and vegetables in the skillet.
09 - Sprinkle shredded cheese and Parmesan over the top.
10 - Cook on the stovetop for 3-4 minutes until edges begin to set.
11 - Transfer the skillet to the preheated oven. Bake for 12-15 minutes, or until the frittata is set in the center and slightly golden.
12 - Let cool for 5 minutes before slicing and serving.