Heat a cast iron skillet over medium-high heat until very hot (about 250F).
Pat the chicken dry, then season generously with salt and freshly ground black pepper.
Place the chicken skin-side down in the hot skillet. Let it sit without moving until the skin crisps and turns golden, about 6 to 8 minutes. This step renders the fat you will use for the rest of the dish.
Flip the chicken and cook the other side until lightly browned, about 4 to 5 minutes.
Add the white parts of the green onions and the garlic slices. Stir gently until the garlic turns fragrant, about 1 minute.
Squeeze in the lemon juice. Cover the skillet, reduce the heat to medium, and cook until the chicken reaches 165F (74C), about 15 minutes.
Transfer the chicken to a plate. Add the butter to the skillet and stir, scraping up the browned bits to form a quick sauce.
Stir in the green onion tops, then return the chicken to the pan and coat with the sauce. Serve immediately with the pan juices.
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