Preheat the oven to 425°F. Grease a large baking sheet with non-stick cooking spray and set aside.
In a medium bowl, whisk together the 1⁄4 cup olive oil, 1 tablespoon of the lemon juice, the Dijon mustard, whole-grain mustard, honey, and garlic. Season with salt and pepper to taste.
Use tongs to dip the chicken thighs in the sauce, coating both sides. Place the thighs on the prepared baking sheet. Discard any remaining sauce.
In a medium bowl, combine the Brussels sprouts and red onion. Drizzle with the remaining 2 tablespoons olive oil and 1 tablespoon lemon juice and toss until well coated. Arrange the sprouts around the chicken on the baking sheet, making sure they aren’t overlapping. Season with salt and pepper.
Roast for 30 to 35 minutes, until the chicken is golden brown and has an internal temperature of 165°F and the Brussels sprouts are crispy. Serve hot.
Recipe by Cotter Crunch