Preheat oven to 325°F. Lightly grease a baking dish or line a rimmed baking sheet with foil.
Add Brussels sprouts, potatoes and red onion to the baking dish. Drizzle with olive oil, sprinkle with salt and pepper, and toss to coat. Spread vegetables around the edges of the dish, leaving room in the center for the turkey roast.
In a small bowl, stir together butter, thyme, lemon peel and salt until well combined.
Pat roast dry. Rub butter mixture on roast to coat, including under skin. Place roast in the center of the baking dish, nestled among the vegetables.
Bake until thermometer inserted in thickest portion registers 170°F, about 1 hour and 20 minutes, stirring vegetables once or twice during roasting. Increase oven temperature to 375°F; bake until skin is golden brown and crisp, 5 to 6 minutes more.
Let stand for 10 to 15 minutes before slicing. Serve turkey with roasted Brussels sprouts, potatoes and red onion.
