1. Sauté the Aromatics and Meat: Brown the sausage and cook down the onion and mushrooms in one tablespoon of butter, seasoning thoroughly with sage, salt, and pepper. Drain off any excess fat and allow to cool slightly.

2. Cube and Layer the Bread: Cut the hearty bread into uniform one-inch cubes and scatter them across the bottom of a greased nine-by-thirteen baking dish.

3. Distribute the Filling: Spread the cooled sausage and vegetable mixture evenly over the bread layer. Top with the two cups of freshly shredded cheddar cheese.

4. Create the Custard: Whisk together the eggs, whole milk, heavy cream, Dijon mustard, and cayenne pepper (if using) until light and homogenous.

5. Soak and Chill: Pour the egg mixture slowly over the layers, gently pressing down on the bread to submerge it. Cover tightly and refrigerate for a minimum of four hours, or preferably overnight.

6. Preheat and Bake: Remove the casserole from the refrigerator thirty minutes before baking. Preheat the oven to three hundred fifty Fahrenheit. Bake uncovered for fifty-five to sixty-five minutes.

7. Rest and Serve: The casserole is done when the center is set and the top is golden. Let it rest for ten minutes before slicing and garnishing with fresh chives.