Using poultry shears, cut along each side of the chicken backbone and remove it. Turn the chicken breast side up and press on the breast bone to flatten the chicken. Using a sharp knife, cut partway through both sides of the joint between the thighs and the drumsticks. Cut partway through the joint between the wings and the breast.
In a bowl, mix all of the remaining ingredients. Turn the chicken breast down and spread it with half of the mustard mixture. Set the chicken in a large skillet skin side up; spread with the remaining mixture.
Set the skillet over high heat and cook the chicken until it starts to brown, 5 minutes. Transfer the skillet to the oven and roast the chicken for 30 minutes, until the skin is browned and the chicken is cooked through. Let the chicken rest for 5 minutes. Transfer the chicken to a cutting board, cut it into 8 pieces and serve.