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 ensuring even cooking.

2. Prepare the Vegetables: In a large bowl, combine the diced sweet potatoes, red onion wedges, and halved Brussels sprouts. Drizzle with 1 tablespoon of olive oil, then sprinkle with 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Toss everything together until the vegetables are evenly coated.

3. Season the Chicken: In a separate medium bowl, place the boneless, skinless chicken thighs. Drizzle with the remaining 1 tablespoon of olive oil. Sprinkle with the dried rosemary, dried thyme, and garlic powder, along with the remaining 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Rub the seasoning into the chicken until each piece is well coated.

4. Arrange on the Sheet Pan: Spread the seasoned vegetables in a single layer on one side of the prepared baking sheet. Place the seasoned chicken thighs in a single layer on the other side of the baking sheet, ensuring they are not overcrowded. This separation helps everything cook properly.

5. Roast Until Cooked Through: Transfer the baking sheet to the preheated oven. Roast for 25-30 minutes, or until the chicken is cooked through (reaching an internal temperature of 165 F (74 C)) and the vegetables are tender and slightly caramelized. You can flip the chicken and stir the vegetables halfway through cooking for more even browning.

6. Serve Warm: Once cooked, carefully remove the sheet pan from the oven. Garnish with fresh chopped parsley, if desired, and serve immediately.