Preheat your oven to 375°F (190°C).
Heat a skillet over medium heat and add olive oil. Once shimmering, add the diced sweet potatoes and sliced fennel.
Sauté the vegetables, stirring occasionally, until they are tender and lightly caramelized, about 8-10 minutes. The kitchen will fill with a warm, sweet aroma.
In a mixing bowl, whisk the eggs until smooth, then stir in the chopped sage and grated Parmesan cheese if using. Season with salt and pepper.
Pour the beaten eggs evenly over the sautéed vegetables in the skillet. Let it cook undisturbed for about 2 minutes until the edges begin to set.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is puffed, golden on top, and the eggs are fully cooked through.
Remove the skillet from the oven and allow the frittata to cool slightly for 5 minutes. Slice into wedges and serve warm, garnished with extra herbs if desired.
