Marinate the chicken: In a large bowl, combine grated garlic, ras el hanout, olive oil, and salt. Add the chicken thighs and use your hands to coat them evenly. Set aside to marinate at room temperature while you prepare the vegetables.
Preheat the oven: Preheat the oven to 425°F (220°C).
Prepare the vegetables: Peel and chop the carrots, turnips, and parsnips into roughly ½ -inch pieces. Peel the red onions and cut each into 4 wedges, then cut the wedges in half.
Arrange on the sheet pan: Lay the chicken thighs skin-side up on a large sheet pan. Add olive oil, salt, and the chopped vegetables to the same bowl used for the chicken, tossing to coat them in the remaining seasoning. Arrange the vegetables around the chicken thighs in a single layer.
Bake: Roast for 45–50 minutes, or until the chicken reaches an internal temperature of at least 165°F (74°C) and the vegetables are tender and caramelized.
Serve: Transfer to a platter and drizzle with any pan juices before serving. Enjoy warm.