Preheat and prep the pan: Preheat the oven to 425°F (220°C). Line a large, rimmed sheet pan with parchment for easy cleanup, or lightly oil it.
Make the lemon-garlic marinade: In a bowl, whisk together olive oil, lemon zest, lemon juice, garlic, oregano, thyme, smoked paprika, red pepper flakes (if using), salt, and pepper.
Season the chicken: Pat the chicken thighs dry with paper towels.
Add them to the bowl and toss to coat well. Let them sit while you prep the vegetables, about 10 minutes.
Toss the vegetables: In a separate bowl, combine potatoes, onion, bell pepper, and green beans or asparagus. Add 1–2 tablespoons of the marinade and a pinch of salt and pepper.
Toss to coat.
Load the sheet pan: Scatter the vegetables and potatoes over the pan in an even layer. Nestle the chicken thighs on top, skin side up. Tuck in a few lemon slices if you like caramelized citrus flavor.
Roast: Place the pan in the oven and roast for 35–45 minutes, or until the chicken skin is crisp, the juices run clear, and an instant-read thermometer reads 165°F (74°C) in the thickest part without touching the bone.
Crisp and finish: If the skin needs extra color, broil for 2–3 minutes, watching closely.
Remove from the oven and rest for 5 minutes so the juices redistribute.
Garnish and serve: Sprinkle with fresh parsley or dill and a final squeeze of lemon for brightness. Serve straight from the pan.