Grease a 13x9 pan and set aside.
In a large skillet, cook sausage over medium high heat until no pink remains. Remove sausage to a plate. Drain fat from pan, leaving about 1/2 tablespoon.
Add diced onions to skillet and saute for about 5 minutes until translucent. Return sausage to skillet and stir into onions for a minute or two. Remove from heat.
In a large bowl, combine eggs, milk, dijon, salt, garlic powder and pepper. Whisk until well combined. Stir in cheese to egg mixture.
Add bread cubes to prepared pan. Evenly spread sausage mixture over bread crumbs. Pour egg mixture over the entire dish.
Cover and place in fridge overnight or for a minimum of 8 hours.
When ready to make, remove dish from fridge and let sit at room temperature for 20 minutes.
Preheat oven to 350 degrees. Bake for 40-50 minutes, uncovered, until the eggs are set and a knife inserted in the center comes out clean. Let sit for 5-10 minutes before cutting.