Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper for effortless cleanup.

In a very large mixing bowl, combine the olive oil, ranch seasoning mix, salt, and black pepper. Whisk these ingredients together until they are well combined.

Add the boneless, skinless chicken thighs, broccoli florets, chopped red bell pepper, baby carrots, and red onion wedges to the bowl with the ranch mixture. Toss everything thoroughly until the chicken and vegetables are evenly coated. Ensure each piece has some seasoning.

Spread the coated chicken and vegetables in a single layer on the prepared baking sheet. Make sure not to overcrowd the pan; this helps the vegetables roast rather than steam. If necessary, use a second baking sheet.

Bake for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C) and the vegetables are tender-crisp. For a slightly crispier finish on the vegetables, you can broil for the last 2-3 minutes, watching them closely to prevent burning.

Remove the baking sheet from the oven. Let the chicken rest for 5 minutes before serving. This helps retain the juices. If using the optional creamy topping, mix 1-2 teaspoons of ranch seasoning into the Greek yogurt or sour cream while the chicken rests. Garnish with fresh chopped parsley, if desired, and serve immediately.