Grease a 9x13-inch dish; to make ahead, prepare through step 4 and refrigerate overnight.
Cook sausage in a large skillet over medium about 8 minutes until browned and crumbled; drain grease and set aside.
Spread bread evenly in dish; top with sausage and cheese.
Whisk eggs, milk, mustard, salt, pepper, garlic powder, and onion powder in a large bowl; pour evenly over bread.
Press gently with a spatula so bread absorbs eggs; cover and refrigerate at least 30 minutes or overnight.
Preheat oven to 350°F; remove casserole from refrigerator while oven heats.
Bake uncovered 40-45 minutes until center sets and top is golden brown; stand 5 minutes, garnish with optional green onions, and serve warm.