Sprinkle salt and pepper all over chicken.

Heat 2 tablespoons unsalted butter in medium cast iron skillet over medium-high heat. Nestle chicken, skin side down in skillet with a single layer with no gaps (if they don't all quite fit, wait until chicken shrinks slightly, then puzzle in the rest.) Cook until meat is opaque around the edges and the chicken is deep golden brown, 6 to 8 minutes.

Turn chicken skin side up and transfer skillet to oven; bake, uncovered until chicken is cooked through, 10 to 15 minutes. Transfer chicken to a plate.

While chicken is browning chop the fennel bulb and fronds, and chop the leek. Place fennel and leek in same skillet. Set over medium heat. Season with salt and pepper and cook, tossing occasionally, until leek is golden around the edges, about 5 minutes.

Add orzo, stirring occasionally, until pasta is darkened to a nutty brown and smells toasty, about 3 minutes. Pour in the wine and cook, stirring, until liquid is evaporated, about 1 minute.

Add chicken broth ½ cup at a time, stirring constantly, and letting broth absorb before adding more, until orzo is tender and the broth is mostly absorbed, but the pan is not dry, 10 to 15 minutes.

Remove skillet from heat and season with salt and pepper, to taste. Mix in 1 tablespoon lemon juice and remaining 1 tablespoon butter, then the fennel fronds.

Pile chicken on top, and finish with lemon zest.