Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or nonstick spray.

Heat a skillet over medium-high. Add the bulk breakfast sausage and cook, breaking it into small pieces, until browned and cooked through (about 6–8 minutes). Drain excess fat on paper towels.

In the same skillet or a separate one, cook the bacon until crispy. Drain on paper towels. Crumble half the bacon for the casserole and reserve the rest to top it before baking.

Spread the thawed shredded potatoes in an even layer in the prepared baking dish. Press lightly so they form a stable base.

Evenly sprinkle half (1 cup) of the cheddar cheese over the potato layer.

Scatter the cooked sausage and the crumbled half of the bacon evenly over the cheese.

In a medium bowl, whisk together the eggs, milk, Dijon mustard, salt, and coarse ground black pepper until smooth and homogenous.

Pour the egg mixture evenly over the layered potatoes, sausage, and cheese. Tap the pan gently to remove air pockets and help the custard settle.

Cover the baking dish tightly with foil or plastic wrap and refrigerate overnight, or at least 6–8 hours.

The next morning, remove the dish from the refrigerator. Uncover and sprinkle the remaining 1 cup of cheddar evenly over the top. Crumble the reserved bacon pieces across the cheese.

Bake uncovered at 350°F (175°C) for 45–55 minutes, or until the center is set and a knife inserted near the middle comes out clean.

Let the casserole rest for 10 minutes before slicing. Garnish with sliced green onions and serve warm.