To prepare stuffing, follow the package directions. Cool slightly.
To prepare the turkey breast, dry the turkey breast thoroughly with a paper towel. Carefully run your hand under the breast skin to create a pocket. Smear the herb butter evenly all over the turkey and under the breast skin. (If the skin is wet, butter won’t stick. So make sure to dry the skin with paper towel well.)
Cut the potatoes into chunks.
Place a trivet in inner pot of your Instant Pot. And place the turkey breast, skin-side down, so that the breast cavity is on top.
Stuff the breast cavity with the prepared stuffing. (Note: You may have some leftover, which is fine. Save it for later.)
Add the potato chunks around the turkey breast.
Pour chicken stock, salt and garlic powder.
Cover the lid and make sure the valve is set to “sealing”.
To pressure cook, set the Instant Pot to Manual/Pressure Cook for 30 minutes (or 10 minutes per lb) on High. (NOTE: It took about 21-25 minutes for my IP to come to pressure.)
Once the time is up, allow the pressure to release naturally for at least 20 minutes, and then carefully turn the valve to venting. Once the pin drops, carefully open the lid.
Check the internal temperature of the turkey breast and make sure it reached at least 165°F.
To broil the turkey breast, transfer the stuffing into a 9x13 (or similar) baking dish and using a pair of kitchen tongs, carefully place the turkey breast over the stuffing. Broil for 3-5 minutes at 500°F to brown the skin. Keep a close eye on it to prevent burning! If one side is browning too quickly, place a sheet of foil over that area and continue to broil until the breast is nice and evenly browned.
To make the mashed potatoes, add milk and butter into the potatoes. (No need to drain the liquid.) Using a potato masher, mash the potatoes into a smooth texture. Transfer onto a serving platter and keep warm.
Clean the inner pot.
To make the gravy, hit "cancel" button and set the Instant Pot to Sauté.
When the display reads “HOT”, add butter (or bacon fat) and melt it. Once the butter is fully melted add flour and roast the flour, whisking constantly, until golden brown with nutty aroma, about 2 minutes.
Slowly pour in chicken stock, while whisking constantly. Once all the stock is added, bring it to a boil, whisking occasionally. Then turn off the Instant Pot and let the gravy simmer for another 5 minute or so. And serve! (Alternatively, you can make the gravy on stovetop in a saucepan, and keep the mashed potatoes in the Instant Pot on “keep warm” setting until ready to serve.)