Preheat the oven to 425 degrees F.
Begin by prepping all of your vegetables as directed in the recipe.
In a large mixing bowl, add the all of the ingredients in the "Frittata" section and whisk together until creamy.
In a cast iron skillet or other oven-safe pan, add in 1 1/2 tbsp of olive or avocado oil, and bring to medium-low heat. Add in the butternut squash and onion, and sauté for 5-7 minutes, stirring frequently. You'll know the butternut squash is ready if it can be pierced with a fork with some difficulty. Meaning, it shouldn't be easy to pierce it with a fork, but it should be able to go through with a little force. Basically, you want the squash to be al dente!
Turn off the heat on the stove. Pour in the frittata mixture to the skillet, and gently stir it a few times, incorporating the butternut squash and onions into the mixture. Then, top the frittata with the 1/4 cup of parmesan cheese.
Bake the frittata for 10-15 minutes, or until the top has just started to brown.
When done, take it out of the oven and let it rest for AT LEAST 20-30 minutes so it can fully set.
Serve, and enjoy!