Preheat your oven to 350°F (175°C).
In a frying pan over medium heat, brown the breakfast sausage until fully cooked, breaking it into crumbles. Drain any excess fat and set aside.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, and Dijon mustard until well combined.
In the prepared baking dish, layer the torn bread pieces at the bottom. Evenly distribute the cooked sausage over the bread.
Pour the egg mixture over the bread and sausage, ensuring everything is soaked well.
Sprinkle the shredded cheddar cheese on top. If using, scatter the chopped green onions for added flavor.
Cover the baking dish with aluminum foil (optional) to avoid excessive browning and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the casserole is set and the top is golden brown.
Let the casserole sit for a few minutes before slicing and serving.
For convenience, the casserole can be prepared the night before and refrigerated overnight before baking it in the morning.