Preheat oven to 375 degrees. Spray a large rimmed baking sheet with non-stick spray. Set prepared pan aside.
Peel, and cube the sweet potatoes. Half the Brussels sprouts and chop the onion. Place the prepared vegetables onto the sheet pan and drizzle with the olive oil. Toss together with your hands to evenly coat.
Mix together the spices; onion powder, garlic salt, paprika and pepper. Sprinkle the spices over the coated vegetables. Bake at 375 for 15 minutes. (Note that the potatoes and vegetables need about 45 total minutes to bake, therefore these go into the oven first without the chicken.)
While the potatoes and vegetables roast, mix together the maple Dijon sauce. In a bowl, whisk together the syrup, vinegar, both mustards, soy sauce and garlic. Set aside.
Once the potatoes and vegetables have roasted for their initial 15 minutes, pull out of the oven. Place the chicken thighs over the top of the vegetables. Spoon the maple Dijon sauce over the top of each chicken thigh. Place the pan back into the oven for another 30 minutes for all ingredients to roast together.
After 30 minutes test the chicken with an instant-read digital thermometer. If the chicken reads 170 degrees, it is done. Let the chicken and vegetables cool for 5-10 minutes before serving.