Turn the oven to 325 and allow your chicken to come to room temperature. Rinse the chicken inside and out (remove gizzards) and pat dry. Cut the chicken into 4 pieces-- two bone-in breasts and two legs (thigh and drumstick together, bone in.) Can also be cut into 8 pieces if you prefer. Salt and pepper all sides. If you'd like a tutorial, check out this video from the NY Times.
Heat up some oil in an enameled cast iron skillet or wide Dutch oven, and brown the chicken pieces in batches: a few minutes per side, starting with the skin side down. You want to try to crisp up the skin here. Remove the chicken to a plate and set aside.
Pour off most of the drippings from the pan so only a thin coating of rendered fat remains. Turn the heat back on, and sauté the mushrooms in the pan for two minutes, and then pour over the sherry and allow it to cook off completely, flavoring the mushrooms. Remove the sherried mushrooms to another bowl/plate.
If your pan looks like it could use some grease to get going, add in a little bit of oil (or some of the rendered fat from the chicken), and then start to cook the onions over medium-low for 4-5 minutes, until beginning to turn translucent. Add in the celery, the red pepper flakes, and the tomato paste and cook for another minute or two.
Pour in the red wine, the entire pint jar of heirloom tomato sauce and bring the sauce up to a boil. Turn off stovetop heat, and nestle in the mushrooms, the sage leaves and chicken (skin side up), ladling a few spoonfuls of sauce on top (especially important on the breast meat.)
Cook for 1 hour in oven, until the sauce begins to thicken and the chicken is super tender. Serve on a bed of lettuce or arugula.