Preheat the oven to 375 degrees.
Heat the oil in a large cast iron skillet (or another oven-safe skillet) over medium-high until it shimmers.
Pat the thighs dry and sprinkle the tops and bottoms with salt and pepper.
Place the thighs skin side down in the hot pan. Don’t move them around so they can get a good sear on the skin, render some fat, and become crispy. Allow them to cook for about 5 minutes per side. The time will be determined by how your burner heats and the size of the chicken thighs.Watch them carefully so they do not burn. Burnt chicken skin isn’t very appetizing. You want a deep golden brown.
Turn the thighs over and cook for another 4 to 5 minutes. Remove them from the pan and set aside to keep warm, uncovered.
Carefully drain off (or use a spoon) to remove all but one tablespoon of the fat.
Lower the heat to medium and add the leeks and garlic. Stir often to keep them from burning. Lower heat if necessary. Allow them to soften and lightly caramelize for about 3 to 4 minutes.
Add the chicken stock and lemon zest to the leeks and garlic and stir, scraping up the browned bits that are on the bottom of the pan.
Add salt and pepper as desired.
Return the chicken thighs to the pan, skin side up.
Place the skillet in the preheated oven and cook the thighs. Begin checking the internal temperature with a probe thermometer after 15 minutes. Continue to cook until the thermometer reads 165 degrees in the thickest part of the thighs.
Remove from the oven. Allow to stand for about 5 minutes and serve warm.