Preheat the oven to 485 F.

In a mixing bowl, whisk together the olive oil, Dijon mustard, garlic and herbes de Provence. Cut each chicken breast into three equal portions. Use a small spatula to spread a small portion of the Dijon-herb mixture over the chicken. About 2 tablespoons of the Dijon-herb mixture should be used in total. Season the chicken with salt and pepper, put it on a plate and into the refrigerator.

Place the onion, carrots, parsnips, and red potatoes in a large baking dish. Season well with salt and pepper. Pour the remaining Dijon-herb mixture all over the vegetables and stir until everything is well-combined. Roast the vegetables (uncovered) in the oven for about 20 minutes, or until the veggies are nearly tender when pierced with a fork.

Remove the dish from the oven and pull the chicken out of the fridge. Nestle the chicken over the roasted veggies and place dish back in the oven (uncovered). Allow the chicken to cook for about 12 minutes, or until the Dijon-herb mixture begins to turn golden brown and the chicken is cooked through. Serve.