Preheat oven to 400°F (200°C). Line a sheet pan with foil or parchment. Pat chicken dry, season with salt and pepper, then brown in a skillet with melted butter for 3-5 minutes per side. Transfer to sheet pan and roast for 15 minutes.
In the same skillet, off heat, add onion, garlic, and Greek seasoning. Stir in asparagus and bell peppers. Mix chicken broth, lemon zest, and juice, then stir into vegetables and sit off heat.
Turn chicken thighs over, add vegetables around, and ladle pan liquid over each piece. Bake until chicken reaches 165°F (74°C), about 15 minutes.
Arrange chicken on a serving dish, surround with vegetables, garnish with lemon slices and parsley. Serve warm.
Notes
For extra flavor, marinate chicken with lemon and garlic beforehand.