Heat olive oil in a large 12-inch oven-safe skillet over medium-high heat. Add the onion to the skillet and cook, stirring occasionally, until onion is softening and turning translucent, approximately 3 minutes. Add the butternut squash to the onion. Stir well to combine. Season with a pinch of salt and pepper.
Stirring frequently, continue to cook the butternut squash until fork tender, but still firm. Add the bell pepper and zucchini to the skillet and stir to combine.
Continue to cook the vegetables for 1-2 minutes before adding the cooked chicken, chopped green onions, cilantro, mint, and parsley.
As the chicken re-heats in the skillet, add the egg whites, milk, and a pinch of salt to a mixing bowl. Whisk vigorously to combine.
Add the butter to the center of the skillet and tilt the pan back-and-forth to distribute the butter across the bottom of the skillet until it is melted fully. Spread the vegetables into an even layer, using a spatula to flatten.
Pour the egg mixture into the skillet directly over the vegetables and chicken, tilting the pan to make sure the eggs have settled evenly.
Reduce heat to medium and cook for 4-5 minutes, or until edges of the eggs have started to set. Carefully crack two eggs directly on top of the frittata (optional) and crumble with casero cheese. Transfer skillet to the oven and bake until eggs have set, approximately 8 minutes.
Remove from oven and allow to cool for 5 minutes before serving. Enjoy!