In a small bowl, whisk together the olive oil, Dijon, lemon zest and juice, garlic, thyme, parsley, salt, pepper, and onion powder.
Transfer 2 tablespoons of the lemon-herb mixture to a small bowl. Cover and refrigerate for later to use with the potatoes.
Place the chicken in a large bowl and add the remaining lemon-herb mixture. Toss well to coat, rubbing some of the marinade underneath the skin and directly onto the flesh of the chicken. For more flavor, cover and refrigerate for 30 minutes to 6 hours.
When ready to cook, preheat oven to 425°F. Lightly coat a large rimmed baking sheet with nonstick cooking spray.
Place the potatoes in a large bowl. Add the reserved lemon-herb mixture and toss well to coat.
Spread the potatoes over the prepared sheet pan, cut-side down. Arrange the chicken skin-side up between the potatoes. Tuck the lemon halves, cut-side down, onto the pan. Cover the sheet pan tightly with foil and roast for 20 minutes.
Remove the foil, turn the potatoes with a spatula, and continue roasting until the chicken reaches 165°F.
Use a spatula to toss the potatoes. Squeeze the roasted lemon over the chicken and potatoes as desired, sprinkle with additional fresh parsley, if desired, and serve.