Preheat oven to 475°F and position rack in the centre. Place a baking sheet pan in the oven to warm up.
Peel, cut into thin wedges, and transfer onion to a large bowl.
Wash and dry beets, carrots, celery, potatoes, and thyme.
Peel and cut beets into thin wedges; add to bowl with onion. Peel and cut carrots diagonally into ¼ inch thick rounds; add to bowl. Cut celery crosswise into ¼ inch thick pieces; add to bowl. Cut potatoes into thin wedges; add to bowl. Slide leaves off most of the thyme stems (saving a couple stems for garnishing); add to bowl.
Drizzle vegetables with olive oil and season with salt, and pepper; toss to coat.
Remove sheet from oven and, using a paper towel, rub with a little olive oil. Place vegetables on sheet and spread out. Place in the oven and roast until vegetables are tender, 40-45 minutes.
Heat a skillet over medium heat.
Roughly chop walnuts.
Add walnuts to skillet and, stirring constantly, cook until toasted, 2-3 minutes.
Peel and mince garlic. In a small bowl, prepare dressing by whisking together garlic, olive oil, vinegar, Dijon, salt, and pepper.
Serve salad on a plate or in a bowl, sprinkle with walnuts, crumble goat cheese over top, drizzle with dressing, and garnish with remaining thyme. Enjoy!
