6 T. unsalted butter, plus 1 T., softened
1 large onion, thinly sliced
3 celery stalks, thinly sliced crosswise
3 garlic cloves, minced
Coarse salt and freshly ground pepper
1 lb. king oyster mushroom or shiitake mushroom, sliced
6 C. day-old rustic-bread cubes (1 inch)
3/4 C. chicken stock
2 T. fresh lemon juice
1 T. finely chopped fresh sage
1 whole chicken (about 4 1/2 lbs.)
Preheat oven to 450°. Melt 2 T. butter in a large ovenproof skillet over high heat. Cook onion, celery and garlic until translucent, 3 to 5 minutes. Season with salt. Remove from skillet. Working in two batches, cook mushrooms in 2 T. butter. Add to onion mixture. Stir in bread, stock, juice and sage. Season with salt and pepper. Return to skillet.
Place chicken, breast side up, on stuffing and tuck wings under. Tie legs together with kitchen twine. Pat chicken dry with paper towels. Rub skin with softened butter and season with salt.
Roast for 30 minutes. Rotate skillet, and roast chicken until juices run clear and an instant-read thermometer inserted into the thickest part of a thigh reaches 165°, about 30 minutes more. Let rest for 10 minutes before serving.