Preheat your oven to 400°F (200°C).
In a large bowl, combine the salt, black pepper, garlic powder, onion powder, and smoked paprika. Add the chicken thighs and toss to coat evenly with the spice mixture.
In a separate bowl, whisk together the melted butter, parsley, Worcestershire sauce, Dijon mustard, lemon juice, and red pepper flakes (if using).
Place the seasoned chicken thighs in a baking dish and pour the butter mixture over the top, ensuring the chicken is well coated.
Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Remove from the oven and let the chicken rest for 5 minutes before serving. Garnish with sliced green onions.