Preheat your oven to 425°F and line a sheet pan with parchment paper.
In a small bowl, whisk together lemon juice, olive oil, minced garlic, fresh thyme, fresh rosemary, salt, and pepper.
Place the chicken breast in the center of the sheet pan. Brush both sides of the chicken with the lemon herb marinade.
Prepare the vegetables: peel and chop the carrot and parsnip into uniform stick shapes, and cut the red onion into wedges.
Toss the vegetables with a little extra salt, pepper, and a drizzle of olive oil if desired on another section of the sheet pan.
Arrange the chicken and vegetables evenly on the sheet pan ensuring there is space for air to circulate.
Roast in the oven for about 20-25 minutes, or until the chicken is cooked through (internal temperature reaches 165°F) and vegetables are tender and slightly caramelized.
Switch the broiler on for the last 2-3 minutes to achieve extra crisp on the chicken skin, watching carefully to avoid burning.
Remove the sheet pan from the oven, let it rest a couple of minutes, then serve warm.