How to Make It
Preheat oven to 425ºF. Place chicken thighs, skin side down, in a large cast-iron skillet lightly coated with cooking spray; sprinkle salt over chicken. Arrange potatoes around chicken. Cook over medium, stirring potatoes occasionally, until chicken skin is browned and crisp, 14 to 16 minutes.
Flip chicken thighs; place skillet in oven. Bake at 425ºF until chicken is done, 8 to 10 minutes. Transfer chicken and potatoes to a plate, keeping as much of the drippings in the skillet as possible.
Add leeks and garlic to skillet. Cook over medium-high, stirring often, until fragrant, 3 to 5 minutes. Add Swiss chard, mustard, and vinegar; cook just until wilted, 2 to 3 minutes. Sprinkle with crushed red pepper. Serve with chicken and potatoes.