Preheat the oven to 375 degrees F (190 degrees C).

Place 3 tablespoons butter in a microwave-safe bowl and heat in a microwave oven until melted, 1 to 2 minutes. Smash garlic cloves with the side of a chef's knife and add garlic to the warm butter. Stir in lemon juice and onion powder. Set aside.

Sprinkle both sides of chicken thighs with salt and pepper. Heat remaining 1 tablespoon butter in a medium-sized oven-safe skillet over medium-high heat. Brown chicken, skin-side down, for 3 to 4 minutes. Flip chicken over and brush skin with lemon-butter mixture. Pour remaining butter mixture into skillet and remove from heat.

Bake in the preheated oven until chicken is no longer pink at the bone and the juices run clear, about 30 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C). Brush skin every 10 minutes with pan juices.

Remove skillet from the oven and place chicken on a serving platter. Drizzle chicken with pan juices and garnish with parsley.