Preheat your oven to 375°F (190°C).
Heat the olive oil in an oven-safe skillet over medium heat until shimmering.
Add the sliced mushrooms to the skillet and sauté until they become fragrant and start to brown, about 5 minutes.
Stir in the diced squash and cook for another 5-7 minutes, until the squash becomes tender and lightly caramelized.
Sprinkle in the minced sage, season with salt and pepper, and stir to combine. Remove the skillet from heat.
In a mixing bowl, whisk the eggs until smooth and slightly frothy.
Pour the beaten eggs evenly over the sautéed vegetables in the skillet, ensuring the mixture spreads out into an even layer.
Sprinkle the shredded cheese evenly over the top, layering flavor and helping it melt during baking.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the eggs are set and the top is golden.
Remove the skillet from the oven and let the frittata rest for a few minutes, allowing it to set and cool slightly.
Slice the frittata into wedges and serve warm, garnished with a sprinkle of extra sage if desired.
