2Using kitchen shears, cut down each side of the backbone.
3Remove the backbone.
4Flip the chicken back over.
5Kick out the thighs and press down on the breast to flatten the chicken.
6Pat the chicken dry with paper towels.
7Gently pull up the skin at the bottom of the breast.
8Run your fingers underneath the skin to create pockets, being careful not to puncture it.
9Repeat the process for the thighs, running your middle finger all the way up.
10Spread the seasoned butter mixture underneath the skin of the chicken.
11Ensure some butter is spread over the thighs as well.
12Spread the seasoned mayonnaise all over the outside of the chicken.
13Trim any excess skin near the neck with kitchen shears.
14Tuck the wings back behind the breast.
15Pour olive oil into a big heavy pan.
16Cut off the hairy end of a head of garlic to expose the cloves.
17Use the cut garlic head to spread the olive oil all over the bottom of the pan.
18Place the garlic head flat side down in the middle of the pan.
19Cut a yellow onion into quarters and arrange it around the garlic in the pan.
20Peel and cut yellow potatoes into big chunks.
21Toss the potatoes with olive oil, salt, and pepper.
22Place the seasoned potatoes around the onions and garlic in the pan.
23Place sprigs of rosemary and thyme over the onions and garlic.
24Place the spatchcocked chicken over the top of the vegetables.
25Flare out the chicken thighs to ensure crispy skin.
26Make sure the legs and wings are inside the pan.
27Cook in a 425-degree Fahrenheit oven on the middle rack for 1 hour and 10 minutes to 1 hour and a half, or until a meat thermometer reads at least 165 degrees Fahrenheit at the thickest part.
28Remove the chicken from the pan and place it on a cutting board to rest.
29Transfer the potatoes from the pan into a bowl.
30Mash the cooked garlic cloves from the pan and add a few to the potatoes.
31Heat 1/2 cup of heavy cream and 2 tablespoons of butter and pour into the potatoes.
32Mash the potatoes until smooth.
33Heat the remaining pan drippings over medium heat.
34Add about 2 tablespoons of flour to the pan (equal parts fat and flour).
35Cook the roux for 2-3 minutes.
36Pour in about a cup of chicken stock and stir until the gravy is smooth and heated through.
37Serve the carved chicken and gravy with the mashed potatoes.