Preheat oven to 400°F (200°C). Pat chicken dry, season with salt and pepper, then brown in butter for 3-5 minutes per side. Transfer to a sheet pan.
Roast chicken in the oven for 15 minutes. Meanwhile, sauté onion, garlic, and Greek seasoning in the same skillet off heat. Add asparagus and bell peppers, then stir in lemon zest, juice, and chicken broth. Let sit off heat.
Turn chicken thighs over, add vegetables around the chicken on the sheet pan, and ladle pan liquid over each piece. Bake for another 15 minutes until cooked through.
Serve chicken with vegetables, garnished with lemon slices and parsley. Serve warm.
Notes
Ensure chicken reaches an internal temperature of 165°F (74°C) for safety.