Pat thighs very dry and unfold them. Season the flesh side with pepper and fennel, then sprinkle salt evenly over both sides.
Rub oil across a cool 12-inch cast-iron skillet and arrange chicken skin-side down in one layer. Set over medium heat and press gently with a spatula for the first minute so the skin lies flat.
Cook for 10 to 14 minutes, reducing heat slightly if the skin darkens too quickly, until fat renders and the skin releases with gentle lifting. Spoon off excess fat, leaving about 1 tablespoon.
Turn thighs and cook for 5 to 8 minutes more until the thickest meat reaches 175°F. Check the meat rather than relying only on the browned skin.
Transfer skin-side up to a plate and rest uncovered for 5 minutes before serving.