Preheat grill to medium heat. Rub the beets with the olive oil and salt and pepper to taste. Place on the grill over indirect heat. Close the lid and roast for 45 minutes to 1 hour, until almost fork tender. Remove from heat and cool completely. Peel the beets, discarding skin, and slice into 1.2-inch (1 cm) thick slices; set aside.
Combine the olive oil, Dijon mustard, whole-grain mustard and vinegar. Mix well and season with salt and pepper to taste; set aside.
Toss the beet slices and onion rounds with the maple syrup and fresh thyme, season with salt and pepper to taste.
Preheat grill to medium-high heat. Arrange the beet slices and onion evenly on a plank. Place plank on grill and close lid. Plank roast for 15 to 20 minutes. Remove plank from grill and cool to room temperature.
Arrange an equal amount of arugula on each of 4 plates. Arrange beets and onion over arugula. Drizzle liberally with mustard dressing and sprinkle with crumbled goat cheese.