Preheat your oven to 400°F (200°C) and lightly grease a ovenproof frying pan or baking dish.
Heat 1 tablespoon of olive oil in the pan over medium heat. Add the sliced apples and cook for about 5 minutes until they start to soften and develop golden edges, releasing a sweet aroma.
Sprinkle the cinnamon over the apples and stir to coat evenly, allowing the mixture to cook for another 2-3 minutes until fragrant and slightly caramelized.
Push the apples to one side of the pan, then add the chopped sage to the cleared space, letting it cook for about a minute until it crisps slightly and releases its aroma.
In a mixing bowl, whisk the eggs until well combined and frothy. Season with salt and pepper and stir in the shredded cheese if using.
Pour the beaten eggs evenly over the apple and sage mixture in the pan, making sure the eggs spread across all ingredients.
Transfer the pan to the preheated oven and bake for 15-20 minutes until the eggs are puffed, set, and lightly golden on top.
Remove the frittata from the oven and let it rest for a few minutes; it will slightly deflate and develop a crisp, golden crust around the edges.
Slice into wedges, plate, and garnish with extra sage if desired. The frittata should be fluffy inside with caramelized apple bits and a crispy top.
Serve warm, enjoying the blend of sweet apples, fragrant sage, and savory eggs—perfect for a cozy fall breakfast or brunch.
