01 - Set the oven to 375°F and allow it to reach the temperature.
02 - Warm olive oil in a 9- to 10-inch oven-safe nonstick skillet over medium heat. Add butternut squash and red onion; cook for 5 minutes, stirring frequently, until they begin to soften.
03 - Incorporate the diced red bell pepper and cook 3 more minutes. Fold in spinach and cherry tomatoes, cooking until spinach wilts, about 2 minutes.
04 - Whisk together eggs, skim milk, dried thyme, oregano, black pepper, and salt in a mixing bowl until well combined.
05 - Pour the egg mixture evenly over the sautéed vegetables in the skillet.
06 - Sprinkle crumbled reduced-fat feta cheese evenly over the top.
07 - Reduce heat to low and cook without stirring for 3 to 4 minutes until the edges start to set.
08 - Transfer the skillet to the preheated oven and bake for 10 to 12 minutes until fully set and lightly golden on top.
09 - Allow the frittata to cool slightly before slicing. Serve warm.