Arrange rack in top third of oven; preheat to 425°.
Season chicken with salt and place, skin side down, in a large, cold cast-iron or ovenproof skillet. Arrange potatoes around chicken. (Placing the chicken in a cold skillet results in the crispiest skin.)
Heat skillet over medium and cook, without moving chicken but turning potatoes occasionally, until chicken skin is browned, crisp, and releases with just a bit of coaxing from a spatula, 14–16 minutes. (The chicken will not be fully cooked at this stage).
Transfer chicken (skin-side up) and potatoes to a plate, leaving as much fat (schmaltz) in skillet as possible.
Reduce heat to medium-low, then add harissa to the schmaltz and cook, stirring occasionally, until fragrant and sizzling, about 30 seconds. Pour off harissa schmaltz into a small bowl; set aside.
Heat 1 tablespoon oil in same skillet over medium. Add leeks, garlic, and three-quarters of sliced fennel and cook, stirring occasionally, until browned and softened, 6-8 minutes.
Return chicken (skin side up, this time) and potatoes to skillet on top of fennel/leek mixture. Transfer skillet to oven and roast until chicken is cooked through and juices run clear when pierced with the tip of a knife, 10–12 minutes.
Meanwhile, toss 1 tablespoon lemon juice, remaining sliced fennel, and 1 tablespoon oil in a small bowl; season with salt.
Add remaining 3 tablespoons lemon juice to reserved harissa schmaltz and stir to combine; season with salt.
Top chicken and potatoes with fennel salad and fronds. Drizzle with harissa dressing. Serve chicken directly from the skillet or transfer to a platter.