Roasting radishes tames their bite and brings out their sweetness.
Preheat the oven to 450°. On a large rimmed baking sheet, toss the turnips, radishes, and oil. Season with salt and pepper. Roast the vegetables, turning them halfway through cooking, until tender and browned in spots, 25 to 30 minutes.
In a large bowl, mix the butter, lemon zest, and garlic; season. Toss with the roasted vegetables and parsley; season.