The previous day, place the chicken breasts and buttermilk in a plastic container with fitting lid. Toss the chicken around until it is completely coated, then place it in the refrigerator to marinate until the next day.
Remove the chicken from the buttermilk (hang on to that, we'll use it later) let it drip for a few minutes and then and sprinkle it with the salt, pepper and dried oregano.
Melt a little bit of cooking fat in a saute pan placed over medium heat. Place the chicken breasts in the hot pan and cook until a nice golden crust forms on both sides, about 5 minutes per side.
While the chicken is cooking, mix the chicken stock, sweet apple cider, Dijon mustard, salt, pepper, chili pepper flakes and chopped fresh thyme together in a large glass measuring cup, and then set that aside.
As soon as the chicken is browned to your liking, remove it to a plate and then throw the grapes, chopped onion and garlic in the skillet. Give that a quick stir and then add the reserved liquid you just made. Stir well, scraping the bottom of the pan to make sure you get all the brown bits. Bring to a boil and then return the chicken to the skillet; Cover partially, lower the heat and simmer for about 20 minutes, or until the chicken is completely cooked through and the onions and grapes are tender.
Mix the tapioca starch with the buttermilk that you used to marinate the chicken and pour that into the skillet. Mix well with a whisk until the sauce thickens and becomes smooth. Don't worry if it appears to be breaking up initially, it will come back together nicely.
Simmer for an additional minute or two and serve, garnished with fresh grapes and chopped parsley, if desired!
Grapes and Buttermilk Chicken https://thehealthyfoodie.com/grapes-and-buttermilk-chicken/