Start by placing the oven rack in the center position and preheat the oven to 425°F. This temperature helps the chicken skin crisp while the vegetables roast evenly. Lightly grease a large sheet pan or roasting pan to prevent sticking.
In a large mixing bowl, combine olive oil, salt, and garlic powder. Add the potatoes, peppers, and onion slices, then toss everything well until evenly coated. Spread the vegetables across the sheet pan in a single layer for even cooking.
Nestle the chicken thighs between the vegetables so the juices can drip onto them while roasting. Scatter the chopped garlic around the pan for extra flavor. Drizzle a little olive oil over the chicken and season generously with salt and black pepper.
Place the sheet pan in the preheated oven and roast for about 40 to 50 minutes. Halfway through cooking, rotate the pan to ensure even browning. The chicken should reach a golden brown color with crispy skin.
The chicken is fully cooked when the juices run clear and the internal temperature reaches 165°F. The potatoes should feel fork-tender, and the peppers should look soft and slightly caramelized. Let everything rest for a few minutes before serving.
Sprinkle fresh parsley over the hot sheet pan right before serving. The herbs add freshness and color to the dish. Serve immediately while the chicken skin is still crispy and the vegetables are warm.