Heat oven to 425°F and position a rack in the upper third.
Pat chicken dry. Season with 3/4 tsp salt, 1/2 tsp pepper, and the smoked paprika.
Whisk olive oil, balsamic, lemon juice, lemon zest, Dijon, honey, garlic, oregano, thyme, and red pepper flakes.
Toss potatoes and red onion with about half the marinade. Spread on a large rimmed sheet pan.
Brush remaining marinade over the chicken. Nestle chicken on top of the vegetables, skin-side up.
Roast 35–40 minutes until potatoes are tender and chicken reaches at least 175°F.
Broil 2–3 minutes to crisp the skin and caramelize the edges, watching closely.
Rest 5 minutes. Spoon pan juices over everything and finish with chopped parsley.