Lightly grease a 9 x 13 inch baking dish.
Trim and discard the rind from the Brie. Cut the cheese into 1-inch cubes and set aside.
In a large skillet over medium heat, cook the sausage until crumbled and no longer pink, 8 to 10 minutes. Drain the grease from the sausage.
Cut the crusts from the bread and place them evenly in the baking dish. Layer the bread slices on top and then add the cooked sausage, Brie, and Parmesan cheese.
In a large bowl, whisk together 5 eggs, 2 cups heavy cream, and the milk, sage, seasoned salt, and mustard. Pour evenly over cheeses. Cover and chill for 8 hours.
When ready to bake, preheat the oven to 350℉.
In a medium bowl, whisk together the remaining 2 eggs and the remaining 1 cup heavy cream. Pour evenly over chilled mixture. Bake casserole for 50 minutes or until set and garnish with green onions if using.