Preheat the oven to 450°F (232°C) and position a rack in the center. Ensure that your baking dish or cast-iron skillet is oven-safe and large enough to accommodate the chicken and vegetables in a single layer.

In a large bowl, combine the minced garlic, chopped thyme leaves, 2 tbs olive oil, 1 tsp kosher salt, and ¼ tsp black pepper. Mix thoroughly to create a fragrant marinade.

Pat the chicken thighs dry with paper towels to remove any excess moisture. This step is essential for achieving crisp skin during roasting. Add the chicken thighs to the bowl with the marinade and coat them evenly, ensuring that the mixture reaches beneath the skin. Allow the chicken to rest for 5 to 10 minutes while preparing the fennel, or refrigerate for up to several hours for a more pronounced flavor.

Trim the fennel bulbs by removing the stalks and any tough outer layers. Slice each bulb into eight wedges. In a separate bowl, toss the fennel wedges with the remaining 1 tbs olive oil, 1 tsp kosher salt, and ¼ tsp black pepper until well coated. Arrange the fennel wedges evenly in a large skillet or baking dish, then layer the orange slices over them.

Place the marinated chicken thighs skin-side up on top of the fennel and orange layers. Scatter the thyme sprigs throughout the dish if using. Roast uncovered in the preheated oven for 35 to 45 minutes, or until the chicken skin is golden brown and the internal temperature reaches at least 165°F (74°C) in the thickest part.

Once cooked, transfer the chicken thighs to serving plates along with roasted fennel and orange slices. Spoon any remaining pan juices over the top for added flavor. Serve immediately with rice, farro, or crusty bread to complete the meal.