Marinate the turkey. Pat the turkey leg completely dry. Stir together the olive oil, Dijon, garlic, onion powder, thyme, salt and pepper into a paste. Rub it all over the turkey, including 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) and roast. Preheat the oven to 350°F (180°C). Place the turkey leg in an 11-inch (28 cm) round ceramic dish or a 13x9-inch (33x23 cm) 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.
Right before it goes into the oven, gently pat the skin with a small amount of olive oil. This extra layer of fat ensures the skin gets incredibly crispy and golden brown while roasting completely uncovered.
Prep the vegetables. Slice the potatoes into ⅜-inch (1 cm) rounds. Peel the celeriac, cut it into quarters and slice into ⅜-½-inch (1-1.5 cm) quarter-rounds. Cut the sweet potato into ½-inch (1.5 cm) rounds. Slice the red onion into ⅜-inch (1 cm) rounds and keep the rings together. Pat everything dry..
Season the vegetables. Toss the potatoes, celeriac and sweet potato with 1½ tablespoon of the olive oil, salt and red chili paprika until lightly coated. Keep the onion separate tossed with oil but keep the pepper away for now.
Add the Vegetables & Finish Roasting After roasting for 60 minutes, carefully take the pan out of the oven. Lift the turkey leg and arrange the seasoned potatoes directly on the bottom underneath it. Next, arrange the celeriac, sweet potatoes, and oiled red onions in distinct separate zones around the meat. Place the whole raw red pepper right in the center next to the turkey. Return the pan to the oven and roast uncovered for another 20 to 30 minutes. Chef's Tip: To get that skin extra crispy and caramelize the edges of your vegetables, turn on your oven's fan (convection setting) and raise the temperature to 195°C (385°F) for this final stretch. Check on it at the 20-minute mark, and turn off the oven once everything reaches your preferred tender, crispy texture.
Finish. Check that the thickest part of the turkey reaches 175°F (80°C). Rest for 10 minutes, then serve.
