Arrange a rack in the center of the oven and preheat to 350°F
Season the chicken generously on both sides with salt and pepper
Heat the oil in a large heavy skillet over medium-high heat
Arrange the chicken skin-side down in the skillet and cook until golden, 6-7 minutes per side
Remove chicken to a plate
Add the leeks and a splash of wine to the skillet and cook, stirring until soft, about 4 minutes
Add the zucchini, salt, pepper and mustard and cook, stirring, until the zucchini releases its liquid, about 4 minutes
Add wine, stock, sage and beans and stir to combine
Bring to a boil and nestle the chicken and any juices in the skillet
Wet and wring out a piece of parchment paper and place on top of chicken, then top with a lid
Cook until the chicken is tender, 35-40 minutes
Remove from the oven and uncover, then drizzle with olive oil
Raise oven temperature to 400°F and return to the oven
Cook uncovered until golden and bubbly, about 10 minutes
Season with salt and sprinkle with gochugaru
