Preheat your oven to 375°F (190°C). Grease an oven-safe skillet or baking dish with a tablespoon of olive oil to prevent sticking and set aside.
Chop the roasted squash into bite-sized cubes, aiming for even pieces for uniform cooking. Arrange the cubes evenly in the prepared skillet.
In a mixing bowl, crack in the eggs and whisk thoroughly until the yolks and whites are fully combined. Add the ground cinnamon, salt, and freshly ground black pepper, then whisk again to incorporate the spices deeply into the mixture.
Pour the seasoned egg mixture evenly over the roasted squash, making sure all the pieces are submerged and well-coated. Gently shake or tilt the skillet to distribute evenly.
Place the skillet into the preheated oven and bake for about 20 to 25 minutes. The eggs will puff up slightly and turn a light golden around the edges as they cook through.
Check for doneness by inserting a toothpick or knife into the center—if it comes out clean and the top is set, the frittata is ready.
Remove the skillet from the oven and let it rest for a few minutes—this helps the frittata set further and makes it easier to slice.
Slice the frittata into portions and serve warm or at room temperature, enjoying the tender eggs balanced with sweet roasted squash and aromatic cinnamon.