1/2 cup milk
2 cloves garlic
3/4 cup freshly grated Parmesan or Pecorino Romano cheese, divided
1 tablespoon extra-virgin olive oil
3/4 pound butternut squash (1 small or half of a medium butternut), peeled and chopped into 1/3-inch cubes
3/4 cup chopped yellow onion
3/4 teaspoon sea salt, divided
Freshly ground black pepper
1 to 2 tablespoons extra-virgin olive oil
16 fresh sage leaves (roughly 1/4 cup), chopped
1Preheat the oven to 425 degrees Fahrenheit. In a large bowl, whisk together the eggs, milk, garlic, 1/4 teaspoon salt and several twists of freshly ground black pepper. Then whisk in about half of the cheese.
2In a 10-inch, well-seasoned cast iron skillet or oven-safe sauté pan, warm 1 tablespoon olive oil over medium heat. Add the chopped onion and stir to coat. Cook for a few minutes, until the onions are starting to turn translucent. Add the squash and 1/2 teaspoon salt and stir. Cover the pan and reduce heat slightly to avoid burning the contents. Cook until the butternut is tender and cooked through, stirring occasionally, about 8 minutes.
3Uncover the pan, raise the heat back to medium and cook until the excess moisture has evaporated and the butternut squash is starting to turn golden on the edges, about 5 to 10 minutes (add another little splash of olive oil if the squash starts sticking to the pan).
4Turn the heat down to low. Arrange the butternut in an even layer in the bottom of the skillet. Whisk the egg mixture one last time and pour it into the pan. Sprinkle the frittata with the remaining cheese. Put the pan in the oven and bake until you can shake the pan (wear oven mitts!) and see that the middle is just barely set, about 14 to 17 minutes.
5While the frittata is baking, fry the sage: Heat oil in a large (read: 12 inch) skillet over medium heat. Once the oil is shimmering, add the sage and toss to coat. Let the sage get crispy (but not brown) before transferring it to a plate covered with a paper towel. Sprinkle the fried sage lightly with sea salt and set it aside.
6Once the frittata is done, sprinkle fried sage on top and let the frittata rest a few minutes before slicing it into 6 large or 8 smaller wedges. Serve!