Preheat your oven to 400°F (200°C). Spread the cubed butternut squash on a baking sheet, drizzle with a tablespoon of olive oil, and sprinkle with chopped sage. Roast for about 20-25 minutes until tender and slightly caramelized.
In a mixing bowl, whisk together the eggs, milk, a pinch of salt and pepper until the mixture is smooth and slightly frothy.
Once the squash is roasted and slightly cooled, gently fold the cubes into the egg mixture, allowing the warm squash to soak up some of the eggs.
Heat your ovenproof skillet over medium heat and add the remaining tablespoon of olive oil. Pour the egg and squash mixture into the skillet, spreading evenly.
Cook for about 3-4 minutes until the bottom begins to set and turns slightly golden, while the top remains runny.
Transfer the skillet to the preheated oven and bake for 12-15 minutes until the top is puffed and golden.
Remove the skillet from the oven and let the frittata rest for a few minutes. Slice into wedges and serve warm, garnished with additional sage if desired.
