Grease a 9x13-inch baking dish with butter or cooking spray and set aside.
In a large skillet over medium heat, cook the breakfast sausage until browned and crumbly, about 8-10 minutes. Drain excess grease and set aside to cool slightly.
Spread half of the bread cubes in the prepared baking dish. Top with half the cooked sausage and half the cheese. Repeat layers with remaining bread, sausage, and cheese.
In a large bowl, whisk together eggs, milk, Dijon mustard, salt, pepper, and garlic powder until well combined.
Pour the egg mixture evenly over the bread layers, pressing down gently with a spatula to help the bread soak up the liquid. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.
When ready to bake, preheat oven to 350°F. Remove strata from refrigerator and let sit at room temperature while oven heats.
Bake uncovered for 45-50 minutes until golden brown on top and set in the center. A knife inserted in the middle should come out clean.
Let cool for 5-10 minutes before slicing. Garnish with sliced green onions if desired and serve warm.