Toss the cubed sweet potatoes, beets, and onion with olive oil, thyme, salt and pepper. Spread them out in a single layer on a baking sheet and roast for about 25-30 minutes, or until the vegetables are tender and slightly caramelized, stirring halfway through for even cooking.
While the vegetables are roasting, prepare the couscous. In a medium saucepan, bring 1 + ¼ cups of water (optionally add a vegetable stock cube, for more flavor) to a boil.
Stir in the couscous and a pinch of salt. Cover the pot, remove from heat, and let it sit for 5 minutes.
After 5 minutes, fluff the couscous with a fork to separate the grains.
For the dressing, whisk together all the ingredients until smooth and well combined.
Add the roasted vegetables and couscous to a serving dish, or individual bowls.
Top with the crumbled goat cheese, chopped parsley, and walnuts (or pecans).