Preheat your oven to 375°F (190°C). Sauté the chopped onion in olive oil in a skillet until translucent and fragrant, about 5 minutes.
Add the roasted root vegetables and chopped sage to the skillet with the onions, stirring gently to combine and warm through, about 2 minutes.
In a mixing bowl, whisk together the eggs, milk or cream, salt, and pepper until fully combined and slightly frothy.
Pour the egg mixture evenly over the vegetables in the skillet, ensuring the ingredients are well covered.
Sprinkle the shredded cheese over the top of the mixture, distributing evenly for a gooey, cheesy crust.
Transfer the skillet to the preheated oven and bake until the eggs are set and the top is golden brown, about 20-25 minutes.
Remove the skillet from the oven and let the frittata rest for 5 minutes to firm up before slicing.
Slice into wedges, serve warm, and enjoy the vibrant flavors with a crispy edge and tender inner texture.