Preheat the oven to 180°C. 

Place all the ingredients for the Garlic Herb Butter into a bowl and mash with a fork to combine well.  

Wash the chicken and pat it dry. 

To butterfly / spatchcock the chicken: Place the chicken breast side down on a chopping board. Cut along the sides of the backbone from tail to the neck until the backbone is out. Make a small insertion through the middle of the bone holding the two breasts and firmly press down to flatten. 

Cover the chicken with the Garlic Herb Butter and massage it into the skin.  

Place the sliced onions, garlic and thyme in a deep baking dish. Pour in the chicken stock, then place the chicken on top. Place on the lid and roast for 45 minutes, until the chicken is cooked through. Remove the lid and continue to roast for 20 minutes until golden brown on top.  

Remove chicken, onion and garlic from the baking dish and let it rest on a platter for 10 minutes. 

Meanwhile, to make the gravy, melt butter in a saucepan over medium heat. Once melted, add the flour and whisk though until it forms a paste. Add the chicken stock and whisk until starts boiling and starts to thicken. Add the chicken juices from the baking dish, the mustard, cream and the lemon juice, mix well and simmer, stirring, until the gravy thickens.  

Cut the chicken in half and serve with the onions and gravy. Garnish with fresh rosemary and thyme leaves.