Preheat your oven to 375°F (190°C). Slice the onions and dice the root vegetables into small, uniform pieces for even cooking.
Heat olive oil in a frying pan over medium heat until it shimmers, then add the onions. Sauté for about 5 minutes until they become fragrant and start to caramelize, releasing a sweet aroma.
Add the diced root vegetables to the pan with a pinch of salt, and cook for another 8-10 minutes until they are tender and lightly browned, stirring occasionally.
Stir in chopped sage and cook for 1 minute to release its aroma, then add the chopped greens to wilt them down for about 2-3 minutes. Remove from heat and let the mixture cool slightly.
Meanwhile, crack the eggs into a large bowl and whisk them until smooth. Season with salt and pepper to taste.
Pour the slightly cooled vegetable mixture into the eggs and gently fold to combine everything evenly.
Transfer the mixture to a greased oven-safe dish or skillet, spreading it out into an even layer. Place it in the oven and bake for 15-20 minutes, until the top is lightly golden and the frittata is puffed and set in the center.
Remove the frittata from the oven and allow it to rest for a few minutes. Slice into wedges and serve warm, showcasing the colorful vegetables and fluffy eggs.