1. Peel turnips, rutabagas, and parsnips and cut into 1/2-inch chunks. Peel onions and cut into 1-inch chunks.
2. In a 10- by 15-inch pan, combine turnips, rutabagas, parsnips, onions, and carrots. Add 2 tablespoons oil and stir to coat vegetables.
3. Bake in a 450° oven until vegetables are tender when pierced, about 1 hour, turning often with a wide spatula to brown evenly.
4. Meanwhile, in a 6- to 8-inch frying pan over medium heat, stir or shake sesame seed until lightly browned, 2 to 4 minutes. Pour from pan into a small bowl.
5. Add remaining tablespoon oil and the vinegar and soy sauce to vegetables; mix well. Pour into a wide bowl. Sprinkle with sesame seed.