Preheat the oven to 375°.
For the potatoes: In a cast add in onions, garlic, and stock. Place the potatoes on top of the mixture taking care to keep them together and uniform and fan them out and layer them. Season with butter, salt and pepper and bake for 60 to 70 minutes.
Preheat the grill to 450° to 550°.
For the chicken: Drizzle the chicken with olive oil and season on all sides with salt and pepper and add to a hot grill skin side down and cook for 20 to 25 minutes making sure it does not burn and does not stick to the grill grates. Turn the heat down and close the grill door.
Flip the chicken over and place on a cooler side of the grill and then turn the heat back up to 450° and cook for 20 to 25 more minutes or until it is cooked through. Rest for 4 to 5 minutes before serving.
Next, add the cream to a medium-size saucepot and cook over medium-low heat while whisking until it becomes very thick, think alfredo sauce. Stir in the mustard and salt and serve..
To Plate: Serve the grilled spatchcock chicken next to the potatoes and mustard sauces and add on optional garnish herbs of chopped chives, parsley, and thyme.