1. Preheat Your Oven and Prepare the Pan: Start by preheating your oven to 400 F (200 C). Line a large baking sheet with parchment paper for easy cleanup. This step is crucial for preventing sticking and making post-dinner tidying a breeze.

2. Prepare the Brussels Sprouts: In a large bowl, toss the trimmed and halved Brussels sprouts with 1 tablespoon of olive oil, 1/4 teaspoon salt, and a pinch of black pepper. Make sure they are evenly coated. This initial seasoning helps them roast beautifully.

3. Prepare the Chicken: Add the cut chicken pieces to the same bowl as the Brussels sprouts (or a separate one if you prefer).

4. Whisk Together the Honey Mustard Glaze: In a small bowl, whisk together the Dijon mustard, honey, apple cider vinegar, garlic powder, remaining 2 tablespoons of olive oil, 1/4 teaspoon salt, and 1/4 teaspoon black pepper until smooth and well combined. This glaze is the star of the show, so ensure all ingredients are thoroughly mixed.

5. Coat and Arrange on the Sheet Pan: Pour about half of the honey mustard glaze over the chicken and Brussels sprouts in the bowl. Toss everything together until the chicken and sprouts are well coated. Spread the chicken and Brussels sprouts in a single layer on the prepared baking sheet, ensuring they are not overcrowded. This allows them to roast rather than steam.

6. Roast to Perfection: Place the baking sheet in the preheated oven and roast for 15 minutes.

7. Flip and Glaze Again: After 15 minutes, carefully remove the sheet pan from the oven. Flip the chicken and Brussels sprouts, then drizzle the remaining honey mustard glaze over everything. Return the pan to the oven and continue roasting for another 5-10 minutes, or until the chicken is cooked through (internal temperature of 165 F / 74 C) and the Brussels sprouts are tender-crisp and slightly caramelized.

8. Serve and Garnish: Once cooked, remove from the oven. If desired, garnish with fresh chopped parsley before serving immediately. The vibrant green of the parsley adds a nice touch of freshness.