Toast and break-up bread in a large bowl. Sprinkle on sage, and salt and pepper to taste. Pour beaten egg over top and toss lightly. Set aside.

In a medium saute pan, melt the stick of butter, add the diced celery, onion, and mushrooms. Saute until onion and celery are clear. Add the mustard and stir in.

Pour onion, celery mix over the bread mix and toss to combine. Pour in stock and stir. Stuffing should look wet when pressed down with a spoon, but there should not be any liquid standing.

Place Stuffing into a LARGE pan and bake at 350 degrees for at least 1 hour. Test center at 1 hour, if not heated through, leave in for 15 minute intervals, checking each time.