Pre-heat oven to 300 degrees F. Spread challah cubes on a cookie sheet and bake for 30 minutes or until dried out, tossing halfway.
Then increase oven to 325 degrees F.
Use butter or margarine to grease a 9x13 inch ceramic or glass casserole pan.
Melt remaining butter or margarine in a large sauté pan over medium heat. Add in carrots, leeks and salt and sauté until vegetables start to soften.
Then add in celery, mushrooms and seasonings. Sauté until all vegetables are slightly soft. Cool slightly.
Mix vegetables with challah bread.
Combine eggs with stock and pour oven bread mixture until well saturated (you may not need all of the liquid).
Bake at 325 degrees F covered in foil for 45 minutes, removing cover for the last
Garnish with fried sage if desired.