1. 8 to 24 hours in advance, make the brine, combine all ingredients in a large stock pot and bring to a boil. Reduce heat and simmer for about 15 minutes until salt and sugar are dissolved.
Remove from heat and add 3 quarts of ice to bring the temperature down. It has to be at 40 degrees before you can add the chicken (tip: this is for health reasons, you don’t want to put the chicken into a danger zone where bacteria can grow, so the brine mixture has to be at 40 degrees or colder).
Place the chicken in the brine, ideally for 24 hours ( you can do this up to 36 hours in advance of cooking). After at least 8 hours in the brine, rinse off the chicken and dispose of the brine.
Place the chicken on a wire rack pan, season it with kosher salt and pepper leave it uncovered in a refrigerator for another 8 hours (this helps to dry out the skin so it can become very crisp when roasted).
When ready to cook, preheat oven to 425, toss the red potatoes with half of the olive oil, granulated garlic and salt and pepper then put them under the rack that the chicken is on and put the entire pan into the oven to roast.
At about 30 minutes carefully remove the chicken and potatoes from the oven, remove the top rack with the chicken and set aside, add the mushrooms (also tossed in the olive oil, granulated garlic, salt and pepper) and toss them with the potatoes, return the chicken on the rack and rotate the pan, continue roasting another 15 – 20 minutes until the skin crisp and golden and the temperature of the chicken when checked at a minimum of 165.
2. While the chicken is roasting. In a saute pan over low/medium heat, toast the mustard seeds (be careful, they will pop!), when they’ve turned a light brown, add the dijon through the thyme, and blend, then add the heavy cream, increase the heat to a soft boil and then reduce and let simmer for about 15-20 minutes, scraping down the sizes.
3. When the chicken is done, let it rest for about 10 minutes, plate potatoes and mushrooms on the bottom of the platter, place chicken on top, and spoon sauce over the chicken, finish with chives.