Preheat and prep. Heat your oven to 425°F (220°C). Line a large sheet pan with parchment or lightly oil it.
Halve the baby potatoes and cut the onion into wedges.
Make the marinade. In a bowl, whisk together olive oil, lemon zest, lemon juice, garlic, oregano, thyme, smoked paprika, Dijon (if using), and half the salt and pepper.
Season the chicken. Pat chicken dry. Add to the marinade and toss to coat. Let sit 10–20 minutes while you season the potatoes.
Season the potatoes. On the sheet pan, toss potatoes and onion with a drizzle of olive oil, the remaining salt and pepper, and a pinch of oregano or thyme.
Spread into an even layer, cut sides down.
Arrange on the pan. Nestle the marinated chicken among the potatoes and onions. Pour any extra marinade over the chicken.
Roast. Bake for 20–25 minutes for thighs, or 18–22 minutes for breasts, until the internal temperature reaches 165°F (74°C) and the potatoes are tender and golden at the edges.
Optional broil. For extra color, broil for 2–3 minutes, watching closely so it doesn’t burn.
Finish and rest. Sprinkle chopped parsley over the hot pan. Let rest 5 minutes so juices settle.
Squeeze a little fresh lemon over the top to brighten everything.
Portion for meal prep. Divide into 4 meal containers. Add an extra wedge of lemon to each for easy reheating refresh.