Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper. Toss the diced root vegetables with a tablespoon of olive oil, salt, and pepper. Spread them out evenly on the baking sheet and roast for 20-25 minutes until tender and caramelized around the edges.
Meanwhile, heat a tablespoon of olive oil in an oven-safe skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are soft and golden brown, about 10 minutes. Add the chopped sage and cook for another minute until fragrant.
Once the vegetables are roasted, add them to the skillet with the caramelized onions and sage. Stir gently to combine and distribute the flavors evenly.
In a mixing bowl, whisk together the eggs, milk, salt, and pepper until smooth and slightly frothy, helping the frittata to be fluffy.
Pour the beaten egg mixture evenly over the roasted vegetables in the skillet. Let it sit for a minute so the eggs begin to set around the edges.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the top is puffed and golden, and a inserted toothpick comes out clean.
Remove the skillet from the oven and let the frittata cool slightly for a few minutes. Slice into wedges showing the colorful, tender interior and crispy topping.
Serve warm, perhaps with a side of greens or a simple vinaigrette for a hearty fall meal or brunch.
