In a large skillet over medium heat, brown the breakfast sausage for about 5-7 minutes, breaking it into crumbles until fully cooked. In the same skillet, add bacon and cook for another 5-6 minutes until crispy. Set half the bacon aside for topping.

Grease a 9x13 inch baking dish. Layer the bottom with shredded potatoes, covering the base evenly. Sprinkle the cooked sausage over the potatoes, followed by half of the cheddar cheese.

In a mixing bowl, whisk together eggs and milk until well combined. Add Dijon mustard, salt, and pepper, mixing until smooth.

Pour the egg and milk custard evenly over the layered ingredients. Top with the remaining cheddar cheese and crumble the reserved bacon on top.

Cover the baking dish tightly with plastic wrap or aluminum foil, and refrigerate overnight or for at least 4 hours.

Preheat your oven to 350°F (175°C). Remove the cover from the casserole and bake for 45-55 minutes, until golden brown and set in the center.

Remove from the oven and let cool for a few minutes before cutting into squares. Serve warm, garnished with chopped green onions.