Heat oven to 425°F. Pat chicken dry, then season with salt, pepper, and half the crushed fennel seed.
Warm olive oil in a large oven-safe skillet over medium-high. Sear chicken skin-side down until deeply golden, 6–8 minutes. Flip and cook 2 minutes more. Transfer to a plate.
Add sliced fennel to the skillet. Cook, stirring, until lightly caramelized, 4–5 minutes. Stir in garlic and remaining fennel seed; cook 30 seconds.
Whisk orange juice, broth, honey, Dijon, zest, red pepper flakes, and thyme. Pour into the pan and scrape up browned bits.
Nestle chicken on top, skin-side up. Tuck in orange slices if using. Transfer skillet to the oven.
Roast until chicken is cooked through and registers 175°F, 20–25 minutes, and sauce slightly thickens.
Return skillet to stovetop over low heat. Swirl in butter until glossy. Rest 5 minutes, then spoon sauce over chicken to serve.