In a large, heavy skillet over medium heat melt the butter, then sauté the onion and celery (and mushroom, if using) until the onion is soft, stirring occasionally.
Stir in the salt, pepper, and sage and cook for an additional 3 minutes.
Place the bread cubes into a large, deep bowl.
Pour 1/4 cup of the butter/celery mixture over the cubes and toss well, then repeat steps until all of the butter mixture is used.
Toss the cubes thoroughly to coat.
(Regarding the optional chicken broth: for dry stuffing, add little or no liquid; for moist stuffing mix in lightly with fork just enough chicken broth to moisten dry crumbs.) Let cool and use as stuffing for the turkey.
We've made this stuffing in the crockpot, as well, adding the chicken broth for moistness.
Adapted from Betty Crocker's Picture Cook Book, circa 1950.
Note: regardling the amount of salt - yes, the amount listed above is correct and is what is listed in the original Betty Crocker recipe. When the butter mixture is first added to the bread, if you taste it at this point it might seem salty (because it is sitting right on the surface of the bread) but remember that the liquid and butter soaks into the bread and redistributes evenly. Also, this recipe, being from the 1950s, is specifically for cooking *inside* the turkey, which a lot of people no longer do, and again the salt will redistribute from the juices in the meat. If you use a crockpot for cooking your stuffing, I'd recommend reducing the amount of salt.
Note #2: again, regarding the salt. You're going to have to decide for yourself about the salt. Having eaten this recipe for every year of my life I know it tastes wonderful as it's written. But that's just me.