Preheat the oven to 425°F. Lightly grease a medium sheet pan and set it aside. Cut the Brussels sprouts in half, or into thirds if they’re larger.
In a small bowl, whisk together 2 tablespoons olive oil, 1 tablespoon lemon juice, Dijon mustard, honey, minced garlic, salt, and pepper until well combined.
Arrange the chicken thighs on the prepared sheet pan. Brush the honey mustard mixture generously over both sides of the chicken.
Place the Brussels sprouts in a medium bowl and toss with the remaining olive oil and lemon juice until lightly coated. Spread them around the chicken in a single layer so they have room to roast.
Bake for about 30 minutes, or until the Brussels sprouts are tender and browned around the edges and the chicken is cooked through. The thickest part of the chicken should reach an internal temperature of 165°F.