Preheat oven to 400°. Rain salt and then some pepper all over chicken. Heat 2 Tbsp. butter in a medium cast-iron skillet over medium-high. Nestle chicken, skin side down, in skillet in a single layer with no gaps (if you can’t quite fit them all, wait until chicken shrinks slightly, then puzzle in the remaining pieces). Cook until meat is opaque around the edges and skin is deep golden brown, 6–8 minutes. Turn chicken skin side up and transfer skillet to oven; bake, uncovered, until chicken is cooked through, 10–15 minutes. Transfer chicken to a plate. Set same skillet over medium; combine fennel bulb and leek in skillet and sprinkle in some salt and pepper. Cook, tossing occasionally, until leek is looking golden around the edges, about 5 minutes. Add orzo and cook until pasta is darkened (it will take on a brown hue) to a nice nutty brown in spots and toasty smelling, about 3 minutes. Pour in wine and cook, stirring, until liquid is evaporated, about 1 minute. Add broth ½ cup at a time, stirring constantly and letting broth absorb before adding more, until orzo is tender and broth is mostly absorbed but pan is not dry, 10–15 minutes. Remove skillet from heat, Taste and add more salt and pepper to your liking; mix in lemon juice and remaining 1 Tbsp. butter, then chopped fennel fronds. Pile chicken on top and finish with lemon zest.