Arrange a rack in top third of oven; preheat to 220C fan. Season the chicken with salt and place, skin side down, in a medium cast iron or other ovenproof skillet.

Arrange the potatoes around the chicken. Heat the skillet over a medium heat and cook, without moving the chicken but turning the potatoes occasionally, until the chicken skin is browned, crisp, and releases with just a bit of coaxing from a spatula (approx. 14 to 16 minutes).

Transfer the chicken, skin side up, and potatoes to a plate, leaving as much fat in the skillet as possible. Let the skillet cool slightly, then add the harissa paste. Return the skillet to a medium heat and cook the harissa, stirring occasionally, until fragrant and sizzling (approx. 30 seconds). Pour off the harissa schmaltz into a small bowl and set aside.

Heat 1 tbsp of the oil in the same skillet over a medium heat. Add the leeks, garlic, and ¾ of the sliced fennel and cook, covered but stirring occasionally, until softened (approx. 8 to 10 minutes). Return the chicken skin side up and potatoes to the skillet. Transfer to the oven and roast until the chicken is cooked through and juices run clear when pierced with the tip of a knife (approx. 10 to 12 minutes).

Meanwhile, toss 1 tbsp of the lemon juice and remaining sliced fennel and 1 tbsp of oil in a small bowl, then season with salt. Add the remaining 2 tbsp of lemon juice to the reserved harissa schmaltz and stir to combine; season with salt.

Transfer the chicken and potatoes to a platter, or serve directly from the skillet. Top with the fennel salad, then the fennel fronds. Drizzle with the harissa dressing.