Butterfly the chicken by opening it up flat. Place the opened chicken on the counter, breast side facing you. Press down firmly with your palms on the breast to flatten the chicken and spread it out. Pat the chicken dry all around using paper towels.

In a bowl, combine the butter, olive oil, and all the marinade ingredients. Mix well to form a paste. Rub the dry chicken all over with the marinade.

Line a large shallow baking tray (oven tray) with parchment paper. Place the chicken on top. Cut the whole head of garlic in half and place it on the tray. Put the chicken in the oven.

Roast for 45 minutes, until the skin is golden brown and the chicken is cooked through. During cooking, baste the chicken two or three times with the juices from the tray.

When the chicken is cooked, remove it from the oven. Tilt the tray and collect all the juices into a small saucepan. Cover the roasted chicken to let it rest while you make the sauce.

Place the saucepan with the juices over medium heat and bring to a boil. Squeeze the roasted garlic cloves to extract the soft garlic pulp and add it to the sauce, discarding the skins. Stir with a whisk. Add mustard and continue stirring.