Preheat oven to 375°F. Take butternut squash and cut off the top and bottom. Slice in half and scoop out the insides, reserving seeds for roasting if you want. Dice into ½" cubes. Toss with 1 tablespoon olive oil and season with salt and pepper. Roast until tender, 25 minutes, flipping halfway through.
In a 6" cast iron skillet, heat butter over medium heat. Add onion and cook for about 5 minutes or until translucent. Add garlic and cook for another minute. Toss in roasted butternut squash.
While cooking onion, whisk together eggs with milk, salt and nutmeg. Add in goat cheese, prosciutto and sage. Add egg mixture to skillet and cook until bottom begins to set, about 5 minutes.
Transfer skillet to oven and cook for another 8-10 minutes if using an 8" skillet or 20-25 minutes if using a 6" inch skillet, or until top is set. Remove from oven and top with some pepitas and sage.
Notes
Use whole full fat dairy which gives the frittata its flavor. You can substitute milk for yogurt or sour cream.
Don’t mess with the egg to dairy ratio or you won’t get the right consistency.
Cast iron skillets are the only way to go for a frittata. Don’t consider anything else.
You know the frittata is done when the center is no longer jiggly and the edges are golden brown.