One day ahead, remove the giblets from the chicken's cavity. Pull any loose fat from around the opening. Rinse the bird inside and out and pat dry. Generously and completely coat the chicken with the yogurt, rubbing it inside the cavity and under the skin. Seal the chicken in a plastic bag and refrigerate overnight.
Heat the oven to 450°. Sprinkle the outside and the cavity of the bird with salt and pepper. Stuff a few mushrooms into the cavity. Put the chicken, breast side up, on a V-shaped rack and set the rack in a roasting pan just larger than the rack. Scatter the rest of the mushrooms in the roasting pan. Roast for 15-20 minutes, reduce the heat to 375° and continue to roast for about 1 more hour for a total of 1 1/4 hours for a 3 lb. chicken. For larger birds, add another 10-15 minutes for each additional pound. The chicken is done when the leg wiggles freely in its joint and when the juices run clear from the thigh when you prick it. A thermometer inserted into the lower meaty part of the thigh should register 170°. Set the chicken on a warm platter, propping up the hindquarters with an inverted saucer, and tent with foil to keep it warm while you make the sauce. Remove the rack from the pan. With a slotted spoon, remove the mushrooms and keep them warm.
Make the sauce from the pan drippings using the following method: tilt the pan and spoon off as much fat as possible. Set the pan over high heat to caramelize all the juices, but be careful not to let them burn. Deglaze with the sherry, scraping up the drippings. Boil until the liquid is just a syrupy glaze, add about 1 1/2 cups of the stock to the pan and boil it down to a bubbling glaze. Repeat with another 1 1/2 cups stock, boiling that down until it's reduced to about 2/3 cup of sauce. Add herbs to taste and adjust the seasoning. Return the mushrooms to the pan and heat through. Carve the chicken and serve the meat drizzled with the mushroom sauce. 4 servings.