Preheat the oven to 425 degrees F.
Toss the carrots, parsnips, beets and radishes together with the thyme, garlic, olive oil, nutmeg, 1 teaspoon salt and pepper to taste in a large bowl. Arrange in 1 layer on a rimmed baking sheet. Roast, stirring once, until tender and golden brown in places, about 25 minutes. Stir in the lemon zest and transfer to a bowl for serving.