Heat oven to 425ºF. Place the parsnips and rutabaga in a large nonstick roasting pan, toss with vegetable oil and kosher salt.
Roast the parsnips and rutabagas for 30 minutes.
Reduce the heat to 350ºF. Add the red onion to the roasting vegetables and stir to combine. Continue to roast the vegetables an additional 40 to 50 minutes.
While the vegetables roast, prepare the vinaigrette: Combine the apple cider vinegar, vegetable oil, thyme, garlic, mustard, sorghum syrup, ginger and nutmeg in a small lidded jar. Cover and shake the jar until the dressing is emulsified.
Add the Brussels sprouts and bread cubes to the roasting vegetables, stir to combine, and roast for an additional 15 minutes. Remove the vegetables from the oven and immediately transfer to a serving bowl. Dress with the vinaigrette while still warm and season with salt and pepper as desired. Serve warm or at room temperature.