Marinate the turkey. Pat the turkey leg completely dry. Mix the marinade: olive oil, softened butter, minced garlic, Dijon, lemon juice, zest, oregano, paprika, pepper, and salt. Rub it over the turkey leg and under the skin where you can. Cover and refrigerate for 8-24 hours or roast right away.
Place the marinated turkey leg in the center of your roasting pan. A white porcelain or ceramic dish works beautifully. Preheat the oven to 350°F (180°C). Place the turkey leg in a 13x9-inch (33x23 cm) or an 11-inch (28 cm) round ceramic baking dish. However, if you prefer your roasted root vegetables to be extra crispy, I highly recommend using a large oven sheet pan instead. A baking sheet provides more surface area, allowing you to spread the vegetables out in a single, even layer so they roast perfectly instead of crowding and steaming.
Toss the whole baby potatoes in a portion of the seasoning. In a large bowl, add the baby potatoes, the olive oil, salt, pepper, garlic powder and oregano. Afterwards, toss the potatoes thoroughly to coat them in part of the garlic-herb mixture. Leave the remaining seasoning in the bowl to use later for the rest of your one-pan turkey dinner.
Nestling the potatoes. Carefully lift the turkey leg from the 13x9-inch (33x23 cm) roasting pan and arrange the seasoned baby potatoes in its place. Arrange the potatoes in an even layer, then lower the turkey leg back down to rest directly on top of them. Roast the dish uncovered.
Prepare and add the butternut squash and Brussels sprouts. While the turkey cooks, prepare the remaining vegetables. Trim the base and remove the outer leaves of the Brussels sprouts, then cut them in half. Peel and slice the butternut squash into ½ to ¾-inch (1–2 cm) thick pieces.
In your original mixing bowl, toss the prepared squash and sprouts in the remaining garlic-herb seasoning. After the turkey has baked for 1 hour and 5 minutes, carefully remove the roasting pan from the oven. Arrange the seasoned vegetables in an even layer around the turkey leg. Drizzle a little extra olive oil over the top of the vegetables, then return the pan to the oven.
Roast 25 more minutes, then turn the oven off. Use the fan setting for the last few minutes if you want extra crispness. Check often so nothing overcooks and ensure that the thickest part of the turkey reads 175°F (80°C), then rest it for 10 minutes before serving.