Step 1

Arrange a rack in top third of oven; preheat to 425°. Season chicken with salt and place, skin side down, in a medium cast-iron or other ovenproof skillet. Arrange potatoes around chicken. 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.

Step 2

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

Step 3

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

Step 4

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

Step 5

Transfer chicken and potatoes to a platter. Top with fennel salad, then fennel fronds. Drizzle with harissa dressing.