If roasting immediately, preheat oven to 475 degrees F (250 degrees C).
In a large mixing bowl, whisk together Arak (3 tablespoon), Olive Oil (1/4 cup), Sunkist Orange (3 tablespoon), Lemon (3 tablespoon), Whole Grain Mustard (2 tablespoon), Honey (3 tablespoon), and Kosher Salt (2 teaspoon).
Season with Freshly Ground Black Pepper (to taste).
If marinating: Place Bone-In Chicken Thighs (8), Clementine (4), Fresh Thyme (to taste), Onion (2) and Fennel Seeds (1 teaspoon) in a large mixing bowl or ziplock bag. Pour the sauce over the top and gently toss everything together with your hands. Turn several times to coat. Marinate chicken for several hours or overnight.
If roasting immediately: Place the Chicken Thighs skin side up in a large roasting pan with clementine slices, thyme, onion quarters and crushed fennel seeds.
Pour the sauce over everything and gently toss together with your hands. Put chicken in the oven.
After 30 minutes, check on the chicken. If the skin is browning too quickly, turn the oven down to 400 degrees F (200 degrees C) and continue roasting until the skin is brown and crisp, 20 to 25 minutes longer.
Transfer chicken and clementines and onion pieces with juices to a serving platter. Let rest 5 to 10 minutes before serving.
Optional: Pour cooking liquid into a small saucepan. Place over medium-high heat, bring to a boil, then simmer until sauce is reduced and you are left with about 1/3 cup. Pour heated sauce over chicken.