Preheat the oven to 425°F (220°C) and position a rack in the center.
Pat the chicken thighs very dry with paper towels to help the skin crisp.
Trim any excess loose skin or fat that might burn during roasting.
In a bowl, combine olive oil, kosher salt, black pepper, garlic powder, smoked paprika, dried thyme, and onion powder.
Rub the spice mixture evenly over the skin and underside of each thigh; drizzle lemon juice if using.
Arrange the thighs skin-side up on a rimmed baking sheet or in a cast-iron skillet, spacing them apart for even air circulation.
Bake for 25–30 minutes, then check color and temperature; larger thighs may need 35–45 minutes total.
If the skin needs more crisping, finish under the broiler for 2–4 minutes while watching closely to avoid burning.
Verify doneness with a meat thermometer — internal temperature should reach 165°F (74°C) at the thickest point.
Remove from the oven and let rest 5 minutes before serving to allow juices to redistribute.