Step 1Prepare the citrus rub: Finely grate the zest of the lemon and set aside. Halve the lemon crosswise, and cut one half into 2 pieces (save the other half for another use). In a small bowl, combine the lemon and orange zest, along with 1 teaspoon fennel seeds, 1 teaspoon dried thyme, 1/4 teaspoon black pepper and 2 teaspoons sea salt, and mix well with a fork. Set aside.
Step 2To prepare the chicken, pat it dry and trim any excess fat. Spread some citrus rub all over the skin and sprinkle inside the cavity. Using your fingers, gently loosen the skin from the breast meat and both thighs to create pockets so you can spread some of the citrus rub there as well. Place the 2 lemon pieces and the garlic inside the cavity and tie the legs with kitchen twine. Place the chicken on a plate and refrigerate, uncovered, for at least 4 hours and up to 24 hours.
Step 3About 21/2 hours before roasting, remove the chicken from the fridge and allow it to come to room temperature. Heat the oven to 475 degrees and oil a 12-inch cast-iron skillet or similar-sized roasting pan.
Step 4In a large bowl, combine the leeks with the remaining 11/2 teaspoons fennel seeds, 1/4 teaspoon salt and 1/4 teaspoon pepper. Drizzle with 2 tablespoons of the olive oil and toss to coat the vegetables. Spread the leeks across the bottom of the skillet. Rub the remaining 1 to 2 tablespoons olive oil across the skin of the chicken and sprinkle with the remaining 1/4 teaspoon salt. Place the chicken, breast-side up, on top of the leeks in the center of the skillet, and tuck the wing tips under the body.
Step 5Roast the chicken for 25 minutes. Reduce the oven temperature to 400 degrees and continue roasting until a thermometer inserted into the thickest part of the thigh reads 165 degrees, 25 to 40 minutes, depending on the size of the bird.
Step 6While the chicken is roasting, cook the barley: In a heavy 2-quart saucepan, combine the water, barley, zest and bay leaf, and bring to a boil. Reduce the heat to maintain a simmer, cover and cook until the grains are tender with a bit of a chew, 30 to 40 minutes. Remove the zest and the bay leaf. Drain and set aside.
Step 7When the chicken is done, remove the skillet from the oven and transfer the chicken to a large cutting board, loosely cover with aluminum foil and set aside to rest for at least 15 minutes.
Step 8Meanwhile, add the cooked grains to the leeks in the skillet and sprinkle with the remaining 1 teaspoon thyme. Pour over the wine — just enough to nicely moisten the grains and vegetables — and stir well to combine. Return the pan to the oven for about 10 minutes, until the grains are heated through, then turn off the oven and keep the door ajar until ready to serve.
Step 9Carve the chicken: Remove the leek and barley mixture from the oven, stir through once, and season to taste with salt and pepper. Serve each portion of leeks and barley with a piece of the chicken.