Pat chicken thighs dry and season generously with salt and Cajun seasoning. Heat olive oil in a skillet over medium-high heat. Sear thighs skin-side down until golden brown (5-7 minutes), flip, and cook 3-4 minutes. Remove and set aside.
Reduce heat to medium. Add butter to the skillet and melt. Sauté garlic and onion until fragrant (1-2 minutes). Pour in chicken broth and any juices from the thighs. Stir to scrape up browned bits. Return thighs to the skillet, reduce heat to low, cover, and simmer 15-20 minutes until fully cooked.
Uncover, flip thighs, and spoon butter-sauce mixture over them. Cook uncovered 2-3 minutes until sauce thickens and skin becomes crisp. Garnish with parsley, green onions, and fresh garlic powder before serving.