Preheat oven to 425 degrees F.
Place baking sheet in oven while the oven is preheating.
To a large bowl add brussels sprouts, sweet potato, red onion, red pepper, 2 tablespoons of olive oil, fresh thyme, salt and pepper. Toss to coat everything.
To a small bowl add course stone ground mustard, dijon mustard, honey, apple cider vinegar, 1 tablespoon of olive oil, garlic clove, fresh thyme, salt and pepper. Whisk to mix everything together. Remove 2 tablespoons of the mixture and set the rest to the side.
Pat chicken dry with a paper towel. Then season with salt and pepper on both sides. Using a pastry brush, evenly brush the 2 tablespoons of the honey mustard mixture across the skin side of all 4 chicken thighs.
When oven is done preheating. Remove the baking sheet from the oven (be careful because it will be hot). Place seasoned chicken thighs, skin side down on baking sheet and spread veggies out around the chicken thighs. Be sure to put them in a single layer and not to crowd the pan. (if you need to use another baking sheet for the veggies do so)
Place the baking sheet in the oven and bake for 18 minutes.
Remove from oven. Flip the chicken over and bake an additional 10-15 minutes. You want the internal temperature of the chicken to be 165 degrees F.
Optional: Put under the broiler for 3-4 minutes for browning.
Serve with the remaining sauce on the side.