From the Chef: 

I often make this recipe at home when I am in a hurry, because splitting and flattening the chicken and cutting between the joints of the leg and the shoul­der reduce the cooking time by half. I use kitchen shears to split the chicken open at the back and to cut the cooked bird into serving pieces and a knife to cut between the joints. 

The mustard crust can be made ahead and even spread on the chicken a day ahead, if you like. I pour the cooked chicken juices into a fat separator with a spout and serve over mashed potatoes, leaving the fat behind. 

Method: 

Preheat the oven to 450 degrees.

For the crust: Mix everything but the chicken in a small bowl. 

Using kitchen shears or a sharp knife, cut alongside the backbone of the chicken to split it open. Spread and press on the chicken with your hands to flatten it. Using a sharp paring knife, cut halfway through both sides of the joints connecting the thighs and drumsticks and cut through the joints of the shoulder under the wings as well. (This will help the heat penetrate these joints and accelerate the cooking process.) 

Put the chicken skin side down on a cutting board and spread it with about half the mustard mixture. Place the chicken fl.at in a large skillet, mustard side down. Spread the remaining mustard on the skin side of the chicken. Cook over high heat for about 5 minutes, then place the skillet in the oven and cook the chicken for about 30 minutes. It should be well browned and dark on top. 

Let the chicken rest in the skillet at room temperature for a few min­utes, then cut it into 8 pieces with clean kitchen shears. Defat the cook­ing juices. If you like, mound some Fluffy Mashed Potatoes on each of four warm dinner plates and place 2 pieces of chicken on each plate. Pour some juice on the mashed potatoes and chicken and serve.