First, you need to preheat your oven to 400 degrees. Then, heat your olive oil in an oven-proof pan or a cast-iron pan over medium-high heat.
Add the red onion, baby spinach, cherry tomatoes, garlic, and red bell pepper to the pan, stirring occasionally. Then, sauté veggies for about 7 minutes.
While vegetables are cooking, combine eggs, milk, red pepper flakes, salt and pepper. Stir in the sun dried tomatoes to the egg mixture.
Pour the whisked eggs on top of the cooked veggies in the cast iron skillet. Use a spatula to press down on the egg mixture and make sure the vegetables are fully coated.
Then, sprinkle the feta cheese on top.
Cook frittata on the stove for 5 minutes.
Transfer into hot oven and cook anywhere from 15-20 minutes. Cooking time may vary. Make sure center is set and not jiggly when pulling out of oven.
Cool frittata to room temperature and then slice into individual slices.
Store in airtight container for up to 5 days in the fridge and enjoy!
