Mise en place (always!). Sauté sliced mushrooms in 1/3 stick of unsalted butter. Cook until slightly softened and mushrooms start to release juices. Remove from pan and set aside.
Brown pork breakfast sausage. Press out grease with a paper towel. While sausage is browning, cube French bread into 3/4" to 1" cubes.
In a large bowl, beat eggs. Whisk in buttermilk and sugar. Add salt, pepper, and ground mustard. Whisk.
Melt remaining 2/3 stick of butter. Pour into bottom of 9" x 13" greased baking dish. Sprinkle half of bread over butter. Press down.
Sprinkle 1/2 of cheese over bread. Spread sausage and mushrooms over cheese.
Sprinkle rest of cheese over sausage and mushrooms. Cover with second half of bread cubes. Press entire mixture down into pan.
Pour egg mixture evenly over casserole. Do this slowly, allowing it to sink it, until casserole dish is full. Reserve remaining egg mixture (if there is any). After a few hours, pour the rest of egg mixture over casserole if it fits.
Cover and refrigerate overnight.
Bake at 325° for 45 minutes. Cover with foil. Cook an additional 45-60 minutes until eggs set.
Notes
You MUST make this the day before. It has to set in the fridge overnight or it won't cook well.