Preheat oven to 375°F.
Remove giblets (if included) from chicken and trim any excess skin. Loosen the skin over the breast and thigh meat and rub thyme, 1 teaspoon pepper and 3/4 teaspoon salt under the skin. Tuck wing tips under and tie legs together with kitchen string. Place breast-side down in a 9-by-13-inch baking pan. Roast the chicken for 45 minutes.
Meanwhile, place sliced leeks in a colander; rinse and drain well. Heat oil in a large skillet over medium heat. Add leeks and cook, stirring often, until softened, about 2 minutes. Add bell pepper; cook for 1 minute. Add cremini, portobello and shiitake mushrooms and cook, stirring often, until the mushrooms release their liquid and it evaporates to a glaze, about 7 minutes. Stir in rosemary, sage and the remaining 1/2 teaspoon pepper and 1/4 teaspoon salt. Cook, stirring, for 1 minute. Transfer the mixture to a large bowl. Stir in bread and broth.
Remove the chicken from the pan. Skim or blot any fat from the pan juices. Add the bread mixture to the roasting pan and spread in an even layer.
Place the chicken, breast-side up, on top of the dressing. Return to the oven and continue roasting until an instant-read thermometer inserted into the thickest part of the thigh without touching bone registers 165°F, about 45 minutes more. Transfer the chicken to a cutting board; let rest for 10 minutes before removing the string and carving. Serve the chicken with the dressing.
TIPS & NOTES:.
Tips: The dark gills found on the underside of a portobello are edible, but if you like you can scrape them off with a spoon.
If you don't have stale bread on hand, toast bread cubes at 250°F until crisped and dry, about 20 minutes.