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