Grease a 9x13-inch dish with butter or cooking spray; set aside.
Cook sausage in a large skillet over medium 8-10 minutes until browned and crumbly; drain grease and cool slightly.
Layer half the bread, half the sausage, and half the cheese in dish; repeat with remaining ingredients.
Whisk eggs, milk, Dijon, salt, pepper, and garlic powder in a large bowl.
Pour evenly over bread; press gently with a spatula to absorb liquid. Cover with plastic wrap; refrigerate at least 4 hours or overnight.
Preheat oven to 350°F; remove strata from refrigerator and stand at room temperature while oven heats.
Bake uncovered 45-50 minutes until golden brown and center sets; a knife inserted in middle should come out clean.
Cool 5-10 minutes before slicing; garnish with optional green onions and serve warm.