- Preheat the oven to 400F
- Cut off and discard the dark green portions of the leeks and slice off the roots. Quarter the leeks lengthwise and rinse thoroughly under running water; pat dry. Place the leeks, cut side down, in a single layer in the bottom of a square baking dish (see Tip). Sprinkle with the salt and pepper and set aside
- In a small bowl, stir together the lemon zest, butter, tarragon, mustard, garlic, and a pinch of salt.
- With your fingers, gently loosen the skin from the chicken. Spoon the butter mixture under the skin of each chicken piece and use your fingers to spread the mixture evenly.
- Place the chicken pieces, skin sides up, on the leeks in the baking dish. Brush the lemon juice over the chicken and season with salt and pepper.
- Bake, uncovered, for 25 minutes.
- Remove the chicken from the baking dish.
- Add the mushrooms and toss to coat with the juices in the dish.
- Place the chicken on top of the vegetables.
- Bake for ten minutes more, spooning the juices over the chicken twice, until the internal temperature of the chicken is 180F and the meat is no longer pink.
- Check the leeks during the last ten minutes of baking and cover loosely with aluminum foil if they are browning too quickly.
- Serve the chicken over the vegetables.
TIP: For easier cleanup, line the baking dish with parchment paper before you add the leeks.