Preheat oven to 400 degrees F. In a large bowl, add eggs, cottage cheese, 1/4 cup Parmesan cheese, 1/2 teaspoon salt, 1/4 teaspoon ground black pepper and red pepper flakes. Whisk and set aside.
Preheat cast iron skillet on medium heat and swirl 1 tablespoon of oil to coat. Add onion and bell pepper, cook for 3-4 minutes, stirring occasionally and seasoning with a sprinkle of salt and pepper. Transfer to a bowl with eggs.
Return skillet to the stove and add another 1 tablespoon of oil to coat. Add mushrooms and cook for 5 minutes, stirring occasionally and seasoning with a sprinkle of salt and pepper.
Add spinach and stir until wilted, 1-2 minutes. Transfer to a bowl with eggs and veggies, and mix well with spatula to combine.
Add remaining 1/2 tablespoon of oil and spread it around to coat the bottom of the skillet evenly. Pour egg mixture, level with spatula, gently spreading all ingredients evenly, and sprinkle with remaining 1/4 cup of Parmesan cheese.
Cook for 1 minute to allow the bottom to set. Then place in the oven and bake for 30-35 minutes or until eggs are puffed and center is no more wet when cut into with a paring knife.
Remove from the oven and let frittata cool for 5 minutes. Then slice into 8 pieces and serve warm or cold.
Notes
Store: Refrigerate leftovers in an airtight container for up to 5 days.
Freeze: Bake, cool completely and slice. Place in an airtight container and freeze for up to 3 months.
Reheat: Reheat individual slices in the microwave for 1 minute, then further 30-40 second increments until warmed through.