Peel the carrots, parsnips, rutabaga, turnip, and potato and cut them into 1/2-inch dice. Combine them on a half sheet pan, toss with 2 tablespoons of the olive oil, and season with salt. Spread the vegetables in a single layer.
Roast, stirring and rotating the pan occasionally, until tender and golden, about 25 minutes.
Meanwhile, heat a larger skillet over medium heat. Heat the remaining 1/2 tablespoon oil. Add the onion and cook, stirring frequently, until tender, about 3 minutes.
Stir in the butter until it melts, then stir in the roasted vegetables. Add the chicken stock and season with salt and pepper. Simmer until the stock thickens and coats the vegetables, about 3 minutes. Stir in the parsley, thyme, and lemon zest. Serve hot or warm.